drm/panfrost: Don't corrupt the queue mutex on open/close

The mutex within the panfrost_queue_state should have the lifetime of
the queue, however it was erroneously initialised/destroyed during
panfrost_job_{open,close} which is called every time a client
opens/closes the drm node.

Move the initialisation/destruction to panfrost_job_{init,fini} where it
belongs.

Fixes: 1a11a88c ("drm/panfrost: Fix job timeout handling")
Signed-off-by: Steven Price <steven.price@arm.com>