When the driver sets up the zpos property it sets the default zpos value
to the HW id of the plane. That is fine as such, but as on many DSS
versions the driver arranges the DRM planes in a different order than
the HW planes (to keep the non-scalable planes first), this leads to odd
initial zpos values. An example is J721e, where the initial zpos values
for DRM planes are 1, 3, 0, 2.

In theory the userspace should configure the zpos values properly when
using multiple planes, and in that sense the initial zpos values
shouldn't matter, but there's really no reason not to fix this and help
the userspace apps which don't handle zpos perfectly. In particular,
some versions of Weston seem to have issues dealing with the planes
with the current default zpos values.

So let's change the zpos values for the DRM planes to 0, 1, 2, 3.

Another option would be to configure the planes marked as primary planes
to zpos 0. On a two display system this would give us plane zpos values
of 0, 0, 1, 2. The end result and behavior would be very similar in this
option, and I'm not aware that this would actually help us in any way.
So, to keep the code simple, I opted for the 0, 1, 2, 3 values.

Fixes: 32a1795f ("drm/tidss: New driver for TI Keystone platform Display SubSystem")
Reviewed-by: Aradhya Bhatia <a-bhatia1@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240213-tidss-fixes-v1-1-d709e8dfa505@ideasonboard.com