drm/i915: disable FBC when more than one pipe is active

We're really supposed to do this to avoid trouble with underflows when
multiple planes are active.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=26987.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: fangxun <xunx.fang@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_drv.h

@@ -215,6 +215,7 @@ enum no_fbc_reason {
 	FBC_MODE_TOO_LARGE, /* mode too large for compression */
 	FBC_BAD_PLANE, /* fbc not supported on plane */
 	FBC_NOT_TILED, /* buffer not tiled */
+	FBC_MULTIPLE_PIPES, /* more than one pipe active */
 };
 
 enum intel_pch {