drm/i915/gvt: make gtt.h self-contained

Add necessary #includes and forward declarations to make the header
compile on its own.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/ed11c982872c7c3bef4f22d13dfa029e18e980a9.1607422863.git.jani.nikula@intel.com