drm/i915: use intel_fb_obj() macros to assign gem objects

Use the macros makes the code cleaner and it also checks for a NULL fb.

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>