arm: mali-dp: Extract mode_config cleanup into malidp_fini

Split out malidp_fini as the opposite of malidp_init. This helps keep
the cleanup paths neat and easier to manage.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>