drm/i915/intel_region_ttm: Provide missing description for 'offset' param

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/i915/intel_region_ttm.c:201: warning: Function parameter or member 'offset' not described in 'intel_region_ttm_resource_alloc'

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230331092607.700644-3-lee@kernel.org

drivers/gpu/drm/i915/intel_region_ttm.c

@@ -181,6 +181,7 @@ intel_region_ttm_resource_to_rsgt(struct intel_memory_region *mem,
 /**
  * intel_region_ttm_resource_alloc - Allocate memory resources from a region
  * @mem: The memory region,
+ * @offset: BO offset
  * @size: The requested size in bytes
  * @flags: Allocation flags
  *