gpu/drm/radeon: fix two memleaks in radeon_vm_init

When radeon_bo_create and radeon_vm_clear_bo fail, the vm->page_tables
allocated before need to be freed. However, neither radeon_vm_init
itself nor its caller have done such deallocation.

Fixes: 6d2f2944 ("drm/radeon: use normal BOs for the page tables v4")
Signed-off-by: Zhipeng Lu <alexious@zju.edu.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>