drm/i915: dont save/restore VGA state for kms

The only thing we really care about that it is off. To do so, reuse
the recently created i915_redisable_vga function, which is already
used to put obnoxious firmware into check on lid reopening.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>