drm/imagination: only init job done fences once

Ensure job done fences are only initialised once.

This fixes a memory manager not clean warning from drm_mm_takedown
on module unload.

Cc: stable@vger.kernel.org
Fixes: eaf01ee5 ("drm/imagination: Implement job submission and scheduling")
Signed-off-by: Brendan King <brendan.king@imgtec.com>
Reviewed-by: Matt Coster <matt.coster@imgtec.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250226-init-done-fences-once-v2-1-c1b2f556b329@imgtec.com


Signed-off-by: Matt Coster <matt.coster@imgtec.com>