drm/v3d: Decouple job allocation from job initiation

We want to allow the IOCTLs to allocate the job without initiating it.
This will be useful for the CPU job submission IOCTL, as the CPU job has
the need to use information from the user extensions. Currently, the
user extensions are parsed before the job allocation, making it
impossible to fill the CPU job when parsing the user extensions.
Therefore, decouple the job allocation from the job initiation.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130164420.932823-8-mcanal@igalia.com