drm/bridge: ti-sn65dsi86: Break GPIO and MIPI-to-eDP bridge into sub-drivers

Let's use the newly minted aux bus to break up the driver into sub
drivers. We're not doing a full breakup here: all the code is still in
the same file and remains largely untouched. The big goal here of
using sub-drivers is to allow part of our code to finish probing even
if some other code needs to defer. This can solve some chicken-and-egg
problems. Specifically:
- In commit 48834e6084f1 ("drm/panel-simple: Support hpd-gpios for
  delaying prepare()") we had to add a bit of a hack to simpel-panel
  to support HPD showing up late. We can get rid of that hack now
  since the GPIO part of our driver can finish probing early.
- We have a desire to expose our DDC bus to simple-panel (and perhaps
  to a backlight driver?). That will end up with the same
  chicken-and-egg problem. A future patch to move this to a sub-driver
  will fix it.
- If/when we support the PWM functionality present in the bridge chip
  for a backlight we'll end up with another chicken-and-egg
  problem. If we allow the PWM to be a sub-driver too then it solves
  this problem.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423095743.v5.9.I3e68fa38c4ccbdbdf145cad2b01e83a1e5eac302@changeid

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