drm/komeda: remove slave_planes property

Properties are uapi like anything else, with all the usual rules
regarding review, testcases, open source userspace ... Furthermore
driver-private kms properties are highly discouraged, over the past
few years we've realized we need to make a serious effort at better
standardizing this stuff.

Again this probably needs multiple pieces to solve this properly:

- To make plane configuration less surprising to userspace you
  propably need to virtualize planes, and reorder which logical plane
  you map to which physical one dynamically. Instead of exposing a
  komeda-specific limitation to userspace and expecting them to dtrt.
  I think msm and rcar-du do that already (and others), if you need
  people to chat with or example code.

- If this is needed for validation, again ->atomic_print_state and the
  infrastructure around that is your friend.

Fixes: 3b9dfa4e ("drm/komeda: Add slave pipeline support")
Cc: Lowry Li (Arm Technology China) <lowry.li@arm.com>
Cc: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Mali DP Maintainers <malidp@foss.arm.com>
Cc: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705121006.26085-2-daniel.vetter@ffwll.ch