drm/amdgpu: cleanup coding style in amdgpu_sync.c file

This is a patch to the amdgpu_sync.c file that fixes some warnings found by the checkpatch.pl tool

Signed-off-by: Jingyu Wang <jingyuwang_vip@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>