drm/i915/gmch: move VGA set state to GMCH code

This is the only user for the GMCH bridge device in display. Move it to
GMCH code.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9bd50fe508246a547cf6e7abfe44ed686a4b3e3a.1673958757.git.jani.nikula@intel.com