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

Reduce include dependencies using forward declarations.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fb54050e03f830a1b29eb14cb37466f39c499cc2.1639142167.git.jani.nikula@intel.com