drm/amdgpu: suppress some compile warnings

Suppress two compile warnings about "no previous prototype".

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Change-Id: I74d029c6ebd8bf6bc08edada7a992d04786330d8