drm/i915: Update size upon return from GEM_CREATE

Since GEM_CREATE is trying to outsmart the user by rounding up unaligned
objects, we used to update the size returned to userspace.
This update seems to have been lost throughout the history.

v2: Use round_up(), reorder locals (Chris)

References: ff72145b ("drm: dumb scanout create/mmap for intel/radeon (v3)")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Janusz Krzysztofik <janusz.krzysztofik@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.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/20190326170218.13255-1-michal.winiarski@intel.com