gallium/vl: use compute preference for all multimedia, not just blit

The compute paths in vl are a bit AMD-specific. For example, they (on
nouveau), try to use a BGRX8 image format, which is not supported.
Fixing all this is probably possible, but since the compute paths aren't
in any way better, it's difficult to care.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111213


Fixes: 9364d66c (gallium/auxiliary/vl: Add video compositor compute shader render)
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 958390a9)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/gallium/auxiliary/util/u_screen.c
	src/gallium/docs/source/screen.rst
	src/gallium/drivers/radeonsi/si_get.c
	src/gallium/include/pipe/p_defines.h