drm/i915: Redisable VGA before the modeset on resume

The VGA plane needs to be disabled before we start doing any
modeset operations on resume.

This should also guarantee that the power well will be enabled
when we call i915_redisable_vga() since it gets explicitly powered on
during resume, and will get powered back off during the modeset
operation if no longer needed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>