drm/i915/gt: Restore coarse power gating

The coarse power gating was disabled as part of commit 2248a283
("drm/i915/gen8+: Add RC6 CTX corruption WA") as a prelude to recover
from the context corruption; the power gating itself has no direct
impact on the RC6 context corruption. However, that recovery scheme was
never implemented due to difficult corner cases, and so we no longer need
to keep the power gating disabled.

Fixes: 2248a283 ("drm/i915/gen8+: Add RC6 CTX corruption WA")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/846


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191231122708.4025916-1-chris@chris-wilson.co.uk