drm/amd/display: check top_pipe_to_program pointer

Clang static analysis reports this error

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:2870:7: warning:
Dereference of null pointer [clang-analyzer-core.NullDereference]
                if
(top_pipe_to_program->stream_res.tg->funcs->lock_doublebuffer_enable) {
                    ^

top_pipe_to_program being NULL is caught as an error
But then it is used to report the error.

So add a check before using it.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -2867,7 +2867,8 @@ static void commit_planes_for_stream(struct dc *dc,
 #endif
 
 	if ((update_type != UPDATE_TYPE_FAST) && stream->update_flags.bits.dsc_changed)
-		if (top_pipe_to_program->stream_res.tg->funcs->lock_doublebuffer_enable) {
+		if (top_pipe_to_program &&
+			top_pipe_to_program->stream_res.tg->funcs->lock_doublebuffer_enable) {
 			if (should_use_dmub_lock(stream->link)) {
 				union dmub_hw_lock_flags hw_locks = { 0 };
 				struct dmub_hw_lock_inst_flags inst_flags = { 0 };