drm/i915: Extract GT render sleep (rc6) management

Continuing the theme of breaking intel_pm.c up in a reasonable chunk of
powermanagement utilities, pull out the rc6 setup into its GT handler.

Based on a patch by Chris Wilson.

Signed-off-by: Andi Shyti <andi.shyti@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20190919143840.20384-1-andi.shyti@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20190927110849.28734-1-chris@chris-wilson.co.uk