drm/i915/fbc: fix FBC_COMPRESSION_MASK on BDW+

Its size is 11:0 instead of 10:0. Found by inspecting the spec. I'm
not aware of any real-world IGT failures caused by this.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477065346-13736-2-git-send-email-paulo.r.zanoni@intel.com