drm/komeda: plane: Remove redundant zpos initialisation

The komeda KMS driver will call drm_plane_create_zpos_property() with an
init value of the plane index.

Since the initial value wasn't carried over in the state, the driver had
to set it again in komeda_plane_reset(). However, the helpers have been
adjusted to set it properly at reset, so this is not needed anymore.

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/20220221095918.18763-10-maxime@cerno.tech