drm/i915/gt: Simplify shmem_create_from_object map_type selection

The object pin created for shmem_create_from_object is just a
single use mapping with the sole purpose of reading the contents
of the whole object in bulk. And the whole source object is also
even a throw-away.  Ergo, the additional logic required by
i915_coherent_map_type can be safely dropped and simplified.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230801153242.2445478-2-jonathan.cavitt@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20230807121957.598420-2-andi.shyti@linux.intel.com