drm/amd/display: Remove unnecessary NULL check in commit_planes_for_stream()

Smatch complains that:

    drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:3369 commit_planes_for_stream()
    warn: variable dereferenced before check 'stream' (see line 3114)

The 'stream' pointer cannot be NULL and the check can be removed.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3478,7 +3478,7 @@ static void commit_planes_for_stream(struct dc *dc,
 				top_pipe_to_program->stream_res.tg,
 				CRTC_STATE_VACTIVE);
 
-			if (stream && should_use_dmub_lock(stream->link)) {
+			if (should_use_dmub_lock(stream->link)) {
 				union dmub_hw_lock_flags hw_locks = { 0 };
 				struct dmub_hw_lock_inst_flags inst_flags = { 0 };