drm/v3d: Consolidate CPU job validation in a function

drm/v3d: Consolidate CPU job validation in a function

All CPU job extension parsers duplicate the same validation procedure:
ensure the extension is attached to a CPU job (not a GPU job) and that
only a single CPU job extension is associated with a given job.

Create a function to consolidate these checks and reduce the boilerplate
across the various CPU job extension handlers. While here, convert the
legacy DRM_DEBUG with a more appropriate drm_dbg().

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patch.msgid.link/20260112-v3d-drm-debug-v2-1-8ef6244c97bb@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>