drm/i915: Restore GT coarse power gating workaround

The workaround to disable coarse power gating is still needed on SKL
GT3/GT4 machines and since the RC6 context corruption was discovered by
the hardware team also on all GEN9 machines. Restore applying the
workaround.

Fixes: c1132367 ("drm/i915: Extract GT render sleep (rc6) management")
Testcase: igt/intel_gt_pm_late_selftests/live_rc6_ctx
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114152621.7235-1-imre.deak@intel.com