drm/i915/gem: sanity check object size in gem_create

Depending on the regions min_page_size we might need to adjust the
object size, ensure this matches our expectations.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114182402.840247-2-matthew.auld@intel.com