drm/i915: Extract GT ring management

Although the ring management is much smaller compared to the other GT
power management functions, continue the theme of extracting it out of
the huge intel_pm.c for maintenance.

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/20191020184139.9145-1-chris@chris-wilson.co.uk