drm/v3d: Simplify job refcount handling

Instead of checking if the job is NULL every time we call the function,
check it inside the function.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130164420.932823-6-mcanal@igalia.com