gbm: Add support for __DRIimage based getBuffers when available

gbm: Add support for __DRIimage based getBuffers when available

This lets us allocate color buffers as __DRIimages and pass them into
the driver instead of having to create a __DRIbuffer with the flink
that requires.

With this patch, we can now run gbm on render-nodes.  A render-node is a
drm device that doesn't support modesetting and all the legacy DRI ioctls.
flink is also not supported, but now that gbm doesn't need flink, we can
run piglit on head-less gbm or head-less GPGPU.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: "10.0" <mesa-stable@lists.freedesktop.org>