Rather than tracking AFBC memory "specially", just use the same codepath
as linear and tiled. Less things to mess up, I figure. This allows us to
use the standard setup_slices() call with AFBC resources, allowing
mipmapped AFBC resources.

Unfortunately, we do have to disable AFBC (and checksumming) in the
meantime to avoid functional regressions, as we don't know _a priori_ if
we'll need to access a resource from software (which is not yet hooked
up with AFBC) and we don't yet have routines to switch the layout of a
BO at runtime.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>