drm/i915: Re-enable FBC WM if the watermark is good on gen6+

If the calculated FBC watermark is no good, we simply disable FBC
watermarks. But we fail to re-enable them later if the calculated
watermark becomes good again. Fix that, but remember to leave FBC
watermarks disabled on ILK since that's required by some workarounds.

v2: Fix checkpatch complaint

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>