drm/v3d: Remove `v3d->cpu_job`

drm/v3d: Remove `v3d->cpu_job`

CPU jobs, like Cache Clean jobs, execute synchronously once the DRM
scheduler starts running them. Consequently, a global `v3d->cpu_job`
variable is unnecessary, as everything is managed within the
`v3d_cpu_job_run()` function.

This commit removes the `v3d->cpu_job` pointer, as it is not needed.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113154741.67520-2-mcanal@igalia.com