drm/i915/fbc: fix the check for already reserved fbc size

The check for previously reserved stolen space size for FBC in
i915_gem_stolen_setup_compression() did not take the compression
threshold into account. Fix this by storing and comparing to
uncompressed size instead.

The bug has been introduced in

commit 5e59f7175f
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Mon Jun 30 10:41:24 2014 -0700

    drm/i915: Try harder to get FBC

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88975
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fbc.c

@@ -78,7 +78,8 @@ static void i8xx_fbc_enable(struct drm_crtc *crtc)
 
 	dev_priv->fbc.enabled = true;
 
-	cfb_pitch = dev_priv->fbc.size / FBC_LL_SIZE;
+	/* Note: fbc.threshold == 1 for i8xx */
+	cfb_pitch = dev_priv->fbc.uncompressed_size / FBC_LL_SIZE;
 	if (fb->pitches[0] < cfb_pitch)
 		cfb_pitch = fb->pitches[0];