drm/xe: Add a missing mutex_destroy to xe_ttm_vram_mgr

Ensure that the mutex is destroyed at fini function.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Bommithi Sakeena <bommithi.sakeena@intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>