turnip: Remove failed command buffer from pool

turnip: Remove failed command buffer from pool

When an error condition occurs during tu_create_cmd_buffer(), the
cmd buffer has already been added to a pool, so the cleanup code should
remove it.

Fixes a crash (assert in tu_device::tu_bo_finish()) in dEQP tests:

dEQP-VK.api.object_management.max_concurrent.command_buffer_primary
dEQP-VK.api.object_management.max_concurrent.command_buffer_secondary

due to pool attempting to destroy an invalid command buffer.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3572>