drm/amdgpu: vcn_v1_0_is_idle() can be static

Fixes: 9b4c412a654c ("drm/amdgpu: Add static CG control for VCN on RV")
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>