With this change we effectively require --enable-gallium-llvm when
building RADV. This should be perfectly safe since the gallium radeonsi
driver already explicitly requires it.

The "gallium" part in --enable-gallium-llvm is about to be removed soon
(not in stable), but until then make sure that things can build.

To reflect the requirement (as opposed to check previously) we rename
llvm_check_version_for to llvm_require_version

Cc: Dave Airlie <airlied@redhat.com>
Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
(cherry picked from commit a66ffcd7)