drm/i915/gvt: make intel_gvt_active internal to intel_gvt

Nobody else uses it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227144408.24345-2-jani.nikula@intel.com