drm/i915/gvt: make mpt.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/73958c4ed1d94aa581acd91f7d7e82785b13d07e.1607422863.git.jani.nikula@intel.com