drm/i915/gem: Only revoke the GGTT mmappings on aperture detiling changes

Only a GGTT mmapping will use the aperture detiling registers, so on a
tiling change for an object, we only need to revoke those mmappings and
not the CPU mmappings (which are always linear irrespective of the tiling).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702163623.6402-1-chris@chris-wilson.co.uk