drm/i915: don't reallocate the compressed FB at every frame

With the current code we just reallocate the compressed FB at every
FBC update: we have X in one frame, then in the other frame we need X
again, but we check "needed < have" instead of "needed <= have".

v2: Rebase after Jani addressed the other problems described in v1.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -253,7 +253,7 @@ int i915_gem_stolen_setup_compression(struct drm_device *dev, int size, int fb_c
 	if (!drm_mm_initialized(&dev_priv->mm.stolen))
 		return -ENODEV;
 
-	if (size < dev_priv->fbc.uncompressed_size)
+	if (size <= dev_priv->fbc.uncompressed_size)
 		return 0;
 
 	/* Release any current block */