drm/i915: Replace i915->gt0 with to_gt(i915)

Quite surprised to see that around i915 there are still i915->gt0
references. Replace them with the to_gt() helper.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230725103330.1041394-1-andi.shyti@linux.intel.com