drm: amdgpu: Remove unneeded semicolon in amdgpu_vm.c

Fix following coccicheck warning:
./drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c:1726:2-3: Unneeded semicolon

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>