st/vdpau: Only call is_video_format_supported hook if needed

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: 5d5b414a7b84 "st/vdpau: fix chroma_format handling in
                     VideoSurfaceQueryGetPutBitsYCbCrCapabilities"

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3848>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3848>