st/vdpau: Only call is_video_format_supported hook if needed

Namely only if *is_supported is true, otherwise the hook result can't affect it.

Avoids

../src/gallium/state_trackers/vdpau/vdpau_private.h:138: FormatYCBCRToPipe: Assertion `0' failed.

with assertions enabled.

Fixes: 5d5b414a "st/vdpau: fix chroma_format handling in VideoSurfaceQueryGetPutBitsYCbCrCapabilities"

/cc @ckoenig