drm/i915/gvt: make mmio_context.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/979ba0ea394e9589f7332b94e91ac200faf995c5.1607422863.git.jani.nikula@intel.com