drm/amdgpu: remove unused parameter in amdgpu_vmid_grab_idle

amdgpu_vm is not used in amdgpu_vmid_grab_idle.

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>