drm/komeda: plane: switch to plane reset helper

komeda_plane_reset() does the state initialisation by copying a lot of
the code found in the __drm_atomic_helper_plane_reset(). Let's switch to
that helper and reduce the boilerplate.

Cc: Brian Starkey <brian.starkey@arm.com>
Cc: "James (Qian) Wang" <james.qian.wang@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220221095918.18763-2-maxime@cerno.tech