drm/i915: Fix not checking cursor and object sizes

Even if the fb is the same we should still check if the sizes are
valid to be set.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>