drm/amdgpu: fix type mismatch error

remaining timeout returned by amdgpu_fence_wait_any can be larger than
max int value, thus the truncated 32 bit value in r ends up being
negative while its original long value is positive.

Signed-off-by: monk.liu <monk.liu@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <jammy.zhou@amd.com>