drm/i915/gen9: Remove WaEnableYV12BugFixInHalfSliceChicken7

Dropping WA because it was for early steppings.

It is fixed in newer preproduction and all production revisions.

v2: add references, updated commit message

References: HSD#2126385, HSD#2131381, BSID#0764
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476977460-28088-1-git-send-email-arkadiusz.hiler@intel.com