drm/amdgpu/vcn: fix no previous prototype warning

Declare 'static', as the function is not intended to be used
outside of this translation unit.

Fixes: 4ed49c95 ("drm/amdgpu/vcn: add unified queue ib test")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>