configure.ac: move enable-gallium-llvm dependency with-gallium-drivers

configure.ac: move enable-gallium-llvm dependency with-gallium-drivers

... to where it's applicable.

Since we effectively made --enable-gallium-llvm mean --enable-llvm with
earlier commits, we need to move the requirement to guard the compnents
added for the LLVM draw.

Otherwise we'll error (as below) when building RADV w/o gallium drivers.

configure: error: --enable-gallium-llvm is required when building radv

v2: Don't remove but move the dependency (Tobias).

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>