drm/amd/display: Add null pointer guards where needed

[Why]
Some functions whose output is typically checked for null are not being
checked for null at several call sites, causing some static analysis
tools to throw an error.

[How]
Add null pointer guards around functions that typically have them at
other call sites.
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Josip Pavic <josip.pavic@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3379,6 +3379,9 @@ static void commit_planes_for_stream_fast(struct dc *dc,
 			&context->res_ctx,
 			stream);
 
+	if (!top_pipe_to_program)
+		return;
+
 	for (i = 0; i < dc->res_pool->pipe_count; i++) {
 		struct pipe_ctx *pipe = &context->res_ctx.pipe_ctx[i];
 
@@ -3978,6 +3981,8 @@ static struct dc_state *create_minimal_transition_state(struct dc *dc,
 	dc->debug.force_disable_subvp = true;
 
 	minimal_transition_context = dc_state_create_copy(base_context);
+	if (!minimal_transition_context)
+		return NULL;
 
 	/* commit minimal state */
 	if (dc->res_pool->funcs->validate_bandwidth(dc, minimal_transition_context, false)) {

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -2460,6 +2460,9 @@ void resource_remove_otg_master_for_stream_output(struct dc_state *context,
 	struct pipe_ctx *otg_master = resource_get_otg_master_for_stream(
 			&context->res_ctx, stream);
 
+	if (!otg_master)
+		return;
+
 	ASSERT(resource_get_odm_slice_count(otg_master) == 1);
 	ASSERT(otg_master->plane_state == NULL);
 	ASSERT(otg_master->stream_res.stream_enc);

drivers/gpu/drm/amd/display/dc/core/dc_stream.c

@@ -447,6 +447,7 @@ bool dc_stream_add_writeback(struct dc *dc,
 	if (dc->hwss.enable_writeback) {
 		struct dc_stream_status *stream_status = dc_stream_get_status(stream);
 		struct dwbc *dwb = dc->res_pool->dwbc[wb_info->dwb_pipe_inst];
+		if (stream_status)
 			dwb->otg_inst = stream_status->primary_otg_inst;
 	}