intel: Allocate miptree for multisample DRI2 buffers

intel: Allocate miptree for multisample DRI2 buffers

Immediately after obtaining, with DRI2GetBuffersWithFormat, the DRM buffer
handle for a DRI2 buffer, we wrap that DRM buffer handle with a region and
a miptree. This patch additionally allocates an accompanying multisample
miptree if the DRI2 buffer is multisampled.

Since we do not yet advertise multisample GL configs, the code for
allocating the multisample miptree is currently inactive.

This patch adds the following fields to intel_mipmap_tree:
    singlesample_mt
    needs_downsample
and the following function stubs:
    intel_miptree_downsample
    intel_miptree_upsample

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>