drm/i915/gen9: Add WaDisableGatherAtSetShaderCommonSlice

Add WaDisableGatherAtSetShaderCommonSlice for all gen9 as stated
by bspec. The bspec told to put this workaround to the per ctx bb.
Initial implementation and subsequent review were done based on
bspec. Arun raised a suspicion that this would belong to indirect bb
instead and he conducted more throughout investigation on the matter
and indeed the documentation was wrong.

v2: Move to indirect_ctx wa bb, as it is correct place (Arun)

References: HSD#2135817
Cc: Arun Siluvery <arun.siluvery@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com> (v1)
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469013973-24104-1-git-send-email-mika.kuoppala@intel.com