drm/i915: Save trip via top-level i915 in a few more places

For gt related operations it makes more logical sense to stay in the realm
of gt instead of dereferencing via driver i915.

This patch handles a few of the easy ones with work requiring more
refactoring still outstanding.

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/20190621070811.7006-30-tvrtko.ursulin@linux.intel.com