va: caps: Don't use image formats for decoded frames.

va: caps: Don't use image formats for decoded frames.

Initially we tried to use the internal color conversion used in i965
and Gallium drivers when decoding. But this approach has showed
limitations and problems.

This patch removes completely the possible color conversion at
decoding, since it show problems with deinterlacing, for example:

gst-launch-1.0 filesrc location=interlaced.mpg2 ! parsebin ! vampeg2dec ! vadeinterlace ! xvimagesink

Allowing only the surface formats when decoding is more stable.

For color conversion is better to do it explicitly with vapostproc.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2531>