drm/i915: Make i915_coherent_map_type GT-centric

Refactor i915_coherent_map_type to be GT-centric rather than
device-centric.  Each GT may require different coherency
handling due to hardware workarounds.

Since the function now takes a GT instead of the i915, the function is
renamed and moved to the gt folder.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Acked-by: Fei Yang <fei.yang@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230801153242.2445478-3-jonathan.cavitt@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20230807121957.598420-3-andi.shyti@linux.intel.com