drm/amdgpu: Fix page fault and kasan warning on pci device remove.

Problem:
When executing echo 1 > /sys/class/drm/card0/device/remove kasan warning
as bellow and page fault happen because adev->gart.pages already freed by the
time amdgpu_gart_unbind is called.

BUG: KASAN: user-memory-access in amdgpu_gart_unbind+0x98/0x180 [amdgpu]
Write of size 8 at addr 0000000000003648 by task bash/1828
CPU: 2 PID: 1828 Comm: bash Tainted: G        W  O      4.18.0-rc1-dev+ #29
Hardware name: Gigabyte Technology Co., Ltd. AX370-Gaming/AX370-Gaming-CF, BIOS F3 06/19/2017
Call Trace:
dump_stack+0x71/0xab
kasan_report+0x109/0x390
amdgpu_gart_unbind+0x98/0x180 [amdgpu]
ttm_tt_unbind+0x43/0x60 [ttm]
ttm_bo_move_ttm+0x83/0x1c0 [ttm]
ttm_bo_handle_move_mem+0xb97/0xd00 [ttm]
ttm_bo_evict+0x273/0x530 [ttm]
ttm_mem_evict_first+0x29c/0x360 [ttm]
ttm_bo_force_list_clean+0xfc/0x210 [ttm]
ttm_bo_clean_mm+0xe7/0x160 [ttm]
amdgpu_ttm_fini+0xda/0x1d0 [amdgpu]
amdgpu_bo_fini+0xf/0x60 [amdgpu]
gmc_v8_0_sw_fini+0x36/0x70 [amdgpu]
amdgpu_device_fini+0x2d0/0x7d0 [amdgpu]
amdgpu_driver_unload_kms+0x6a/0xd0 [amdgpu]
drm_dev_unregister+0x79/0x180 [drm]
amdgpu_pci_remove+0x2a/0x60 [amdgpu]
pci_device_remove+0x5b/0x100
device_release_driver_internal+0x236/0x360
pci_stop_bus_device+0xbf/0xf0
pci_stop_and_remove_bus_device_locked+0x16/0x30
remove_store+0xda/0xf0
kernfs_fop_write+0x186/0x220
__vfs_write+0xcc/0x330
vfs_write+0xe6/0x250
ksys_write+0xb1/0x140
do_syscall_64+0x77/0x1e0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f66ebbb32c0

Fix:
Split gmc_v{6,7,8,9}_0_gart_fini to postpone amdgpu_gart_fini to after
memory managers are shut down since gart unbind happens
as part of this procedure

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>