drm/komeda: Remove layer_split 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:

- Instead of expecting userspace to compute this (and duplicating
  modeset code), the kernel driver should compute when it's necessary
  to enable layer_split mode to make a configuration possible. I.e. in
  komeda_plane_atomic_check() first try komeda_build_layer_data_flow()
  and if that fails, try komeda_build_layer_split_data_flow(), and set
  dflow.en_split accordingly. Assuming I understand somewhat correctly
  what this does.

- If this is needed for validation then you want a debugfs file to
  force this one way or the other, or alternatively  use
  ->atomic_print_state to dump such hidden driver-private state.
  Depends upon how you do your validation ofc.

Fixes: a407a650

 ("drm/komeda: Add layer split 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/20190705121006.26085-4-daniel.vetter@ffwll.ch