drm/amd/amdgpu/amdgpu_device: Provide suitable description for param 'xcc_id'

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

 drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:516: warning: Function parameter or member 'xcc_id' not described in 'amdgpu_mm_wreg_mmio_rlc'

Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>