drm/i915: Return correct size for rotated views

Currently object size is returned for the rotated VMA size which can be
bigger than the rotated view itself. Since the binding code pads all
excess size with scratch pages the only minor issue with this is wasting
some GGTT space, but still feels nicer to fix and report the real size.

v2: Rebase for tracking size in bytes instead of pages.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>