drm/bridge: ps8640: Drop the ability of ps8640 to fetch the EDID

In order to read the EDID from an eDP panel, you not only need to
power on the bridge chip itself but also the panel. In the ps8640
driver, this was made to work by having the bridge chip manually power
the panel on by calling pre_enable() on everything connectorward on
the bridge chain. This worked OK, but...

...when trying to do the same thing on ti-sn65dsi86, feedback was that
this wasn't a great idea. As a result, we designed the "DP AUX"
bus. With the design we ended up with the panel driver itself was in
charge of reading the EDID. The panel driver could power itself on and
the bridge chip was able to power itself on because it implemented the
DP AUX bus.

Despite the fact that we came up with a new scheme, implemented in on
ti-sn65dsi86, and even implemented it on parade-ps8640, we still kept
the old code around. This was because the new scheme required a DT
change. Previously the panel was a simple "platform_device" and in DT
at the top level. With the new design the panel needs to be listed in
DT under the DP controller node. The old code allowed us to properly
fetch EDIDs with ps8640 with the old DTs.

Unfortunately, the old code stopped working as of commit 102e80d1
("drm/bridge: ps8640: Use atomic variants of drm_bridge_funcs"). There
are cases at bootup where connector->state->state is NULL and the
kernel crashed at:
* drm_atomic_bridge_chain_pre_enable
* drm_atomic_get_old_bridge_state
* drm_atomic_get_old_private_obj_state

The crash went away at commit 4fb912e5 ("drm/bridge: Introduce
pre_enable_prev_first to alter bridge init order") which added a NULL
check. However, even though we were no longer crashing the end result
was that we weren't actually powering the panel on when we thought we
were. Things could end up working (despite warning splats) if
userspace was persistent and tried to get the mode again, but it
wasn't great.

A bit of digging was done to see if there was an easy fix but there
was nothing obvious. Instead, the only device using ps8640 the "old"
way had its DT updated so that the panel was no longer a simple
"platform_deice". See commit c2d94f72 ("arm64: dts: mediatek:
mt8173-elm: Move display to ps8640 auxiliary bus") and commit
113b5cc06f44 ("arm64: dts: mediatek: mt8173-elm: remove panel model
number in DT").

Let's delete the old broken code so nobody gets tempted to copy it or
figure out how it works (since it doesn't).

NOTE: from a device tree "purist" point of view, we're supposed to
keep old device trees working and this patch is technically "against
policy". Reasons I'm still proposing it anyway:
1. Officially, old mt8173-elm device trees worked via the "little
   white lie" approach. The DT would list an arbitrary/representative
   panel that would be used for power sequencing. The mode information
   in the panel driver would then be ignored / overridden by the EDID
   reading code in ps8640. I don't feel too terrible breaking DTs that
   contained the wrong "compatible" string to begin with. NOTE that
   any old device trees that _didn't_ lie about their compatible will
   still work because the mode information will come from the
   hardcoded panels in panel-edp.
2. The only users of the old code were Chromebooks and Chromebooks
   don't bake their DTs into the BIOS (they are bundled with the
   kernel). Thus we don't need to worry about breaking someone using
   an old DT with a new kernel.
3. The old code was broken anyway. If someone wants to fix the old
   code instead of deleting it then they have my blessing, but without
   a proper fix the old code isn't useful.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616165517.v2.1.I7b8f60b3fbfda068f9bf452d584dc934494bfbfa@changeid