glx/dri3: Add additional check for gpu offloading case

glx/dri3: Add additional check for gpu offloading case

Checks blitImage is implemented.
Initially having the __DRIimageExtension extension
at version 9 at least meant blitImage was supported.
However some implementation do advertise version >= 9
without implementing it.

CC: 10.5 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Axel Davy <axel.davy@ens.fr>