drm/i915: Introduce to_gt() helper

To allow further refactoring and abstract away the fact that GT is
stored inside i915 private.
No functional changes.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211214193346.21231-3-andi.shyti@linux.intel.com