drm/amdgpu/virt: fix double kfree on bo_va

bo_va is being kfree'd twice, once in the call to amdgpu_vm_bo_rmv
and then a short while later. Fix this double free by removing
the 2nd kfree.

Detected by CoverityScan, CID#1399524 ("Double Free")

Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>