drm/amd/display: Fix potential null pointer dereference in dc_dmub_srv

Fixes potential null pointer dereference warnings in the
dc_dmub_srv_cmd_list_queue_execute() and dc_dmub_srv_is_hw_pwr_up()
functions.

In both functions, the 'dc_dmub_srv' variable was being dereferenced
before it was checked for null. This could lead to a null pointer
dereference if 'dc_dmub_srv' is null. The fix is to check if
'dc_dmub_srv' is null before dereferencing it.

Thus moving the null checks for 'dc_dmub_srv' to the beginning of the
functions to ensure that 'dc_dmub_srv' is not null when it is
dereferenced.

Found by smatch & thus fixing the below:
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:133 dc_dmub_srv_cmd_list_queue_execute() warn: variable dereferenced before check 'dc_dmub_srv' (see line 128)
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:1167 dc_dmub_srv_is_hw_pwr_up() warn: variable dereferenced before check 'dc_dmub_srv' (see line 1164)

Fixes: 028bac58 ("drm/amd/display: decouple dmcub execution to reduce lock granularity")
Fixes: 65138eb7 ("drm/amd/display: Add DCN35 DMUB")
Cc: JinZe.Xu <jinze.xu@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Josip Pavic <josip.pavic@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Qingqing Zhuo <Qingqing.Zhuo@amd.com>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dc_dmub_srv.c

@@ -128,7 +128,7 @@ bool dc_dmub_srv_cmd_list_queue_execute(struct dc_dmub_srv *dc_dmub_srv,
 		unsigned int count,
 		union dmub_rb_cmd *cmd_list)
 {
-	struct dc_context *dc_ctx = dc_dmub_srv->ctx;
+	struct dc_context *dc_ctx;
 	struct dmub_srv *dmub;
 	enum dmub_status status;
 	int i;
@@ -136,6 +136,7 @@ bool dc_dmub_srv_cmd_list_queue_execute(struct dc_dmub_srv *dc_dmub_srv,
 	if (!dc_dmub_srv || !dc_dmub_srv->dmub)
 		return false;
 
+	dc_ctx = dc_dmub_srv->ctx;
 	dmub = dc_dmub_srv->dmub;
 
 	for (i = 0 ; i < count; i++) {
@@ -1169,7 +1170,7 @@ void dc_dmub_srv_subvp_save_surf_addr(const struct dc_dmub_srv *dc_dmub_srv, con
 
 bool dc_dmub_srv_is_hw_pwr_up(struct dc_dmub_srv *dc_dmub_srv, bool wait)
 {
-	struct dc_context *dc_ctx = dc_dmub_srv->ctx;
+	struct dc_context *dc_ctx;
 	enum dmub_status status;
 
 	if (!dc_dmub_srv || !dc_dmub_srv->dmub)
@@ -1178,6 +1179,8 @@ bool dc_dmub_srv_is_hw_pwr_up(struct dc_dmub_srv *dc_dmub_srv, bool wait)
 	if (dc_dmub_srv->ctx->dc->debug.dmcub_emulation)
 		return true;
 
+	dc_ctx = dc_dmub_srv->ctx;
+
 	if (wait) {
 		if (dc_dmub_srv->ctx->dc->debug.disable_timeout) {
 			do {