drm/amdgpu:fix NULL pointer access during drv remove

NULL pointer is because original logic will step into
set_pde_pte() even after the gart.ptr is freed due to
there are twice gart_unbind() on all gart area.

also, there are other minor fixes:
1,since gart_init only create dummy page, the corresponding
gart_fini shouldn't do more like unbinding all GART, this is
unnecessary because in driver fini stage all GART unbinding
had already been done during each IP's SW_FINI (GMC's
SW_FINI is the last one called), so remove the step
for the GART unbinding in gart_fini().

2,gart_fini() is already invoked during each GMC IP's gart_fini
routine,e.g. gmc_vx_0_gart_fini(), so no need to manually
call it during ttm_fini().

3,amdgpu_gem_force_release() should be put ahead of
amdgpu_vm_manager_fini()

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -2506,7 +2506,6 @@ void amdgpu_device_fini(struct amdgpu_device *adev)
 	/* evict vram memory */
 	amdgpu_bo_evict_vram(adev);
 	amdgpu_ib_pool_fini(adev);
-	amdgpu_fw_reserve_vram_fini(adev);
 	amdgpu_fence_driver_fini(adev);
 	amdgpu_fbdev_fini(adev);
 	r = amdgpu_fini(adev);