drm/i915: enable WaDisableDopClkGating for skl

This WA is required when decoupled frequencies for slice and unslice
are enabled. This disables DOP clock gating for skl.

v2: enable the WA for all gen9 platforms (not just for SKL GT4 where
    the hang issue is originally reported) to avoid rare hangs (David)
v3: as per WaDatabase, enable it only for SKL (Rodrigo)

Cc: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Praveen Paneri <praveen.paneri@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1501781530-8186-1-git-send-email-praveen.paneri@intel.com