drm/i915: make intel_frontbuffer.h self-contained

This will be helpful in the follow-up work. No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9a19d43987006e4249b335f3d843da43c998376c.1554461791.git.jani.nikula@intel.com