drm/i915: Set the SR01 "screen off" bit in i915_redisable_vga() too

From BSpec / SR01 - Clocking Mode:
"The following sequence must be used when disabling the VGA plane.
 Write SR01 to set bit 5 = 1 to disable video output.
 Wait for 100us.
 Disable the VGA plane via Bit 31 of the MMIO VGA control."

So simply call i915_disable_vga() from i915_redisable_vga().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>