panfrost: Don't leak temporary descriptors array

As found by Coverity:

>>>     CID 1462596:  Resource leaks  (RESOURCE_LEAK)
>>>     Variable "descriptors" going out of scope leaks the storage it points to.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4724>