drm: mali-dp: add custom reset hook for planes

Now that we are using a custom plane state in mali-dp, we need a
bespoke reset that takes into account the larger structure.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
[Updated commit message]
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>