auxiliary/vl: use vl_*_screen_create stubs when building w/o platform

auxiliary/vl: use vl_*_screen_create stubs when building w/o platform

Provide a dummy stub when the user has opted w/o said platform, thus
we can build the binaries without unnecessarily requiring X11/other
headers.

In order to avoid build and link-time issues, we remove the HAVE_DRI3
guards in the VA and VDPAU state-trackers.

With this change st/va will return VA_STATUS_ERROR_ALLOCATION_FAILED
instead of VA_STATUS_ERROR_UNIMPLEMENTED. That is fine since upstream
users of libva such as vlc and mpv do little error checking, let
alone distinguish between the two.

Cc: Leo Liu <leo.liu@amd.com>
Cc: Guttula, Suresh <Suresh.Guttula@amd.com>
Cc: mesa-stable@lists.freedesktop.org
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>