drm/amdgpu: amdgpu_kiq_reg_write_reg_wait() can be static

Fixes: d790449835e6 ("drm/amdgpu: use kiq to do invalidate tlb")
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>