drm/i915: move i915_vgacntrl_reg() where needed

Reduce clutter from i915_drv.h.

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/d30a79d008b875f708f5acf7924f9ca8ab06b575.1556809195.git.jani.nikula@intel.com