drm/amdgpu: fix no previous prototype warning

add static prefix for vangogh_set_apu_thermal_limit function

Signed-off-by: Kun Liu <Kun.Liu2@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202303010827.c2N0yBGT-lkp@intel.com


Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>