damage: Simplify DamageUnregister

damage: Simplify DamageUnregister

You can only register one drawable on a given damage, so there's no
reason to require the caller to specify the drawable, the damage is
enough.  The implementation would do something fairly horrible if you
_did_ pass mismatched drawable and damage, so let's avoid the problem
entirely.

v2: Simplify xf86RotateDestroy even more [anholt]

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>