i965/miptree: Allocate mcs_buf for an image's CCS

i965/miptree: Allocate mcs_buf for an image's CCS

This code will disable actually creating these buffers for the scanout,
but it puts the allocation in place.

Primarily this patch is split out for review, it can be squashed in
later if preferred.

v2:
assert(mt->offset == 0) in ccs creation (as requested by Topi)
Remove bogus is_scanout check in miptree_release

v3:
Remove is_scanout assert in intel_miptree_create. It doesn't work with
latest codebase - not sure it ever should have worked.

v4:
assert(mt->last_level == 0) and assert(mt->first_level == 0) in ccs setup
(Topi)

v5 (Jason Ekstrand):
 - Base the decision to allocate a CCS on the image modifier

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chad Versace <chadversary@chromium.org>