drm/amdgpu: destroy mutex when tearing down ttm buffer entity

Pair mutex_init() with mutex_destroy() on the GART window allocation
failure path and in amdgpu_ttm_buffer_entity_fini(). This is a no-op
without CONFIG_DEBUG_MUTEXES, but it keeps lockdep from seeing a live
mutex after the entity is freed or re-initialized.

Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 03ac40aeed034a0a0d5824ad26f8fbe1f6463698)
Cc: stable@vger.kernel.org