drm/i915: make intel_gvt.h self-contained

Add the minimal includes/declarations to make the header self-contained,
and ensure it stays that way.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626144020.2155-12-jani.nikula@intel.com