Revert "damage: Make damageRegionProcessPending take a damage not a drawable"

The commit message makes the assertion that the code below damage is not
allowed to change whether there's a damage monitor for the drawable.
That turns out not to be the case! exa's mixed code, at least, will
create and destroy a damage in PrepareAccess. The destroy path can then
be catastrophic, as damageRegionProcessPending will attempt to
RegionEmpty memory from the middle of a freed block.

I'd wanted that invariant for performance, but faster isn't worth
broken, so revert it. I think what exa's doing is reasonable, so the
better way to improve performance for the unmonitored case is to either
revisit dynamically wrapping into the GC, or inline damage into dix.

This reverts commit 4e124203.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1389886


Signed-off-by: Adam Jackson <ajax@redhat.com>