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 5e59f7175f96550ede91f58d267d2b551cb6fbba
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>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index b6b02f3..ddfa701 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -772,7 +772,7 @@
 };
 
 struct i915_fbc {
-	unsigned long size;
+	unsigned long uncompressed_size;
 	unsigned threshold;
 	unsigned int fb_id;
 	enum plane plane;