drm/xe: fix pvc unload issue

Currently, unload pvc driver will generate a null dereference
and the call stack is as below.

[ 4850.618000] Call Trace:
[ 4850.620740]  <TASK>
[ 4850.623134]  ttm_bo_cleanup_memtype_use+0x3f/0x50 [ttm]
[ 4850.628661]  ttm_bo_release+0x154/0x2c0 [ttm]
[ 4850.633317]  ? drm_buddy_fini+0x62/0x80 [drm_buddy]
[ 4850.638487]  ? __kmem_cache_free+0x27d/0x2c0
[ 4850.643054]  ttm_bo_put+0x38/0x60 [ttm]
[ 4850.647190]  xe_gem_object_free+0x1f/0x30 [xe]
[ 4850.651945]  drm_gem_object_free+0x1e/0x30 [drm]
[ 4850.656904]  ggtt_fini_noalloc+0x9d/0xe0 [xe]
[ 4850.661574]  drm_managed_release+0xb5/0x150 [drm]
[ 4850.666617]  drm_dev_release+0x30/0x50 [drm]
[ 4850.671209]  devm_drm_dev_init_release+0x3c/0x60 [drm]

There are a couple issues, but the main one is due to TTM has only
one TTM_PL_TT region, but since pvc has 2 tiles and tries to setup
1 TTM_PL_TT each tile. The second will overwrite the first one.

During unload time, the first tile will reset the TTM_PL_TT manger
and when the second tile is trying to free Bo and it will generate
the null reference since the TTM manage is already got reset to 0.

The fix is to use one global TTM_PL_TT manager.

v2: make gtt mgr global and change the name to sys_mgr

Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Vivi, Rodrigo <rodrigo.vivi@intel.com>
Signed-off-by: Bruce Chang <yu.bruce.chang@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>