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>

drivers/gpu/drm/i915/intel_display.c

@@ -8936,8 +8936,7 @@ void i915_redisable_vga(struct drm_device *dev)
 
 	if (I915_READ(vga_reg) != VGA_DISP_DISABLE) {
 		DRM_DEBUG_KMS("Something enabled VGA plane, disabling it\n");
-		I915_WRITE(vga_reg, VGA_DISP_DISABLE);
-		POSTING_READ(vga_reg);
+		i915_disable_vga(dev);
 	}
 }