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

drivers/gpu/drm/i915/intel_drv.h

@@ -486,6 +486,7 @@ struct cxsr_latency {
 #define to_intel_encoder(x) container_of(x, struct intel_encoder, base)
 #define to_intel_framebuffer(x) container_of(x, struct intel_framebuffer, base)
 #define to_intel_plane(x) container_of(x, struct intel_plane, base)
+#define intel_fb_obj(x) (x ? to_intel_framebuffer(x)->obj : NULL)
 
 struct intel_hdmi {
 	u32 hdmi_reg;