vaapipostproc: suppress direction and colorimetry critical messages when no VPP entrypoint
As pointed out in !279 (merged), some drivers don't provide a VAEntrypointVideoProc.
!279 (merged) fixes the critical messages for invalid display but does not address the critical messages for video direction and colorimetry:
** (gst-launch-1.0:20481): CRITICAL **: 18:11:29.263: gst_vaapi_filter_get_video_direction: assertion 'filter != NULL' failed
** (gst-launch-1.0:20481): CRITICAL **: 18:11:29.263: gst_vaapi_filter_set_colorimetry: assertion 'filter != NULL' failed