drm/amdgpu: unmap and remove csa_va properly

Root PD BO should be reserved before unmap and remove
a bo_va from VM otherwise lockdep will complain.

v2: check fpriv->csa_va is not NULL instead of amdgpu_mcbp (christian)

[14616.936827] WARNING: CPU: 6 PID: 1711 at drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c:1762 amdgpu_vm_bo_del+0x399/0x3f0 [amdgpu]
[14616.937096] Call Trace:
[14616.937097]  <TASK>
[14616.937102]  amdgpu_driver_postclose_kms+0x249/0x2f0 [amdgpu]
[14616.937187]  drm_file_free+0x1d6/0x300 [drm]
[14616.937207]  drm_close_helper.isra.0+0x62/0x70 [drm]
[14616.937220]  drm_release+0x5e/0x100 [drm]
[14616.937234]  __fput+0x9f/0x280
[14616.937239]  ____fput+0xe/0x20
[14616.937241]  task_work_run+0x61/0x90
[14616.937246]  exit_to_user_mode_prepare+0x215/0x220
[14616.937251]  syscall_exit_to_user_mode+0x2a/0x60
[14616.937254]  do_syscall_64+0x48/0x90
[14616.937257]  entry_SYSCALL_64_after_hwframe+0x63/0xcd

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>