drm/i915: split out i915_ptr_util.h

Move pointer related utilities from i915_utils.h to a separate new
i915_ptr_util.h header. Clean up related includes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/3cd06aa2483e68f19401292e9d4c28bf2977fce5.1757582214.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>