drm/i915: Introduce intel_fb_obj() macro

Add an intel_fb_obj() macro that returns the GEM object associated with
a DRM framebuffer.  This macro is safe to call on NULL framebuffers (a
NULL object pointer will be returned in this case).

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>