drm/ttm: Fix a memory leak

Free the memory on failure.
Also no need to re-alloc memory on retry.

Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219042547.44855-1-xinhui.pan@amd.com


Reviewed-by: Christian König <christian.koenig@amd.com>
CC: stable@vger.kernel.org # 5.11
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>