va: Add HEVC decoding support

va: Add HEVC decoding support

This add HEVC decoding support into the new VA plugin. This implementation has
been tested using the ITU comformance test (through fluster). It fails all
MAIN10 tests, as this is not implemented yet along with the following:

CONFWIN_A_Sony_1 (looks fine, but md5sum is incorrect)
PICSIZE_A_Bossen_1 (height too high)
PICSIZE_B_Bossen_1 (same)
VPSSPSPPS_A_MainConcept_1 (parser issue)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1714>