drm/i915: Unify SLICE_UNIT_LEVEL_CLKGATE w/a for cnl

drm/i915: Unify SLICE_UNIT_LEVEL_CLKGATE w/a for cnl

gem_workarounds reports that the SLICE_UNIT_LEVEL_CLKGATE write isn't
sticking. Commit 0a60797a0efb ("drm/i915: Implement
ReadHitWriteOnlyDisable.") presumes that SLICE_UNIT_LEVEL_CLKGATE is a
masked register in the context image, but commit 90007bca6162
("drm/i915/cnl: Introduce initial Cannonlake Workarounds.") lists it as
an ordering unmasked register. The masked write will be losing the
default settings if we trust the original commit. That gem_workarounds
reports the value is lost entirely is more worrying though -- but it
clearly suggests that it is not a masked register in the context image,
so unify both w/a to use the original rmw.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103705
Fixes: 0a60797a0efb ("drm/i915: Implement ReadHitWriteOnlyDisable.")
References: 90007bca6162 ("drm/i915/cnl: Introduce initial Cannonlake Workarounds.")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171111100336.11020-1-chris@chris-wilson.co.uk
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>