drm/i915/icl: WaEnableFloatBlendOptimization

Enables blend optimization for floating point RTs

v2: Rebased on top of the WA refactoring
v3: Added References (Mika)

References: HSDES#1406393558
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1527285939-20113-5-git-send-email-oscar.mateo@intel.com