drm/v3d: Don't allow two multisync extensions in the same job

Currently, two multisync extensions can be added to the same job and
only the last multisync extension will be used. To avoid this
vulnerability, don't allow two multisync extensions in the same job.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130164420.932823-7-mcanal@igalia.com