drm/i915: Introduce gt_init_mmio

We already call 2 gt-related init_mmio functions in driver_mmio_probe
and a 3rd one will be added by a follow-up patch, so pre-emptively
introduce a gt_init_mmio function to group them.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200708003952.21831-6-daniele.ceraolospurio@intel.com