drm/i915: Compartmentalize ring buffer creation

Continuing on the theme of compartmentalizing the code better to make
future split between gt and display in global i915 clearer.

v2:
 * Pass in ggtt instead of gt. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-29-tvrtko.ursulin@linux.intel.com