drm/i915/fbc: avoid intel_frontbuffer.h include with declaration

Reduce include dependencies using forward declarations.
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fb54050e03f830a1b29eb14cb37466f39c499cc2.1639142167.git.jani.nikula@intel.com

drivers/gpu/drm/i915/display/intel_fbc.h

@@ -8,8 +8,7 @@
 
 #include <linux/types.h>
 
-#include "intel_frontbuffer.h"
-
+enum fb_op_origin;
 struct drm_i915_private;
 struct intel_atomic_state;
 struct intel_crtc;