drm/i915/gen9: Add WaFbcTurnOffFbcWatermark

drm/i915/gen9: Add WaFbcTurnOffFbcWatermark

According to bspec this prevents screen corruption when fbc is
used.

v2: This workaround has a name, use it (Ville)
v3: remove bogus gen check on ilk/vlv wm path (Ville)

References: HSD#2135555, HSD#2137270, BSID#562
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-25-git-send-email-mika.kuoppala@intel.com