drm/amd/display: fix dereferencing possible ERR_PTR()

This patch fixes static checker warning caused by
"36cc549d5986: "drm/amd/display: disable CRTCs with
NULL FB on their primary plane (V2)"

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4854,6 +4854,9 @@ static int dm_atomic_check_plane_state_fb(struct drm_atomic_state *state,
 			return -EDEADLK;
 
 		crtc_state = drm_atomic_get_crtc_state(plane_state->state, crtc);
+		if (IS_ERR(crtc_state))
+			return PTR_ERR(crtc_state);
+
 		if (crtc->primary == plane && crtc_state->active) {
 			if (!plane_state->fb)
 				return -EINVAL;