drm/amd/display: Check return code for CRC drm_crtc_vblank_get

[Why]
The call to drm_crtc_vblank_get can fail if vblank is disabled and
we try to increment the reference.

Since drm_crtc_vblank_get internally drops the reference when it fails
it means the subsequent drm_crtc_vblank_put(...) when closing the file
drops a zero reference.

This was found via igt@kms_plane@pixel-format-pipe-A-planes.

[How]
Check the return code and return it on failure.

We wouldn't have been able to enable CRC reading anyway since vblank
wasn't enabled.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -106,6 +106,7 @@ int amdgpu_dm_crtc_set_crc_source(struct drm_crtc *crtc, const char *src_name)
 	struct drm_dp_aux *aux = NULL;
 	bool enable = false;
 	bool enabled = false;
+	int ret;
 
 	enum amdgpu_dm_pipe_crc_source source = dm_parse_crc_source(src_name);
 
@@ -176,7 +177,10 @@ int amdgpu_dm_crtc_set_crc_source(struct drm_crtc *crtc, const char *src_name)
 	 */
 	enabled = amdgpu_dm_is_valid_crc_source(crtc_state->crc_src);
 	if (!enabled && enable) {
-		drm_crtc_vblank_get(crtc);
+		ret = drm_crtc_vblank_get(crtc);
+		if (ret)
+			return ret;
+
 		if (dm_is_crc_source_dprx(source)) {
 			if (drm_dp_start_crc(aux, crtc)) {
 				DRM_DEBUG_DRIVER("dp start crc failed\n");