drm/i915/gvt: only include intel_gvt.h where needed

i915_drv.c is the only caller.

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/20200227144408.24345-3-jani.nikula@intel.com