drm/i915: Fix VGA_DISP_DISABLE check

The VGACNTRL register contains a bunch of other stuff besides
the VGA_DISP_DISABLE bit. When we write the register we always set those
other bits to zero, so normally the current check would work.

However on HSW disabling and re-enabling the power well will reset the
VGACNTRL register to its default value, which has several of the other
bits set as well.

So only look at the VGA_DISP_DISABLE bit when checking whether the VGA
plane needs re-disabling.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -10663,7 +10663,7 @@ void i915_redisable_vga(struct drm_device *dev)
 	    (I915_READ(HSW_PWR_WELL_DRIVER) & HSW_PWR_WELL_STATE_ENABLED) == 0)
 		return;
 
-	if (I915_READ(vga_reg) != VGA_DISP_DISABLE) {
+	if (!(I915_READ(vga_reg) & VGA_DISP_DISABLE)) {
 		DRM_DEBUG_KMS("Something enabled VGA plane, disabling it\n");
 		i915_disable_vga(dev);
 		i915_disable_vga_mem(dev);