drm/bridge: parade-ps8640: Wait for HPD when doing an AUX transfer

Unlike what is claimed in commit f5aa7d46 ("drm/bridge:
parade-ps8640: Provide wait_hpd_asserted() in struct drm_dp_aux"), if
someone manually tries to do an AUX transfer (like via `i2cdump ${bus}
0x50 i`) while the panel is off we don't just get a simple transfer
error. Instead, the whole ps8640 gets thrown for a loop and goes into
a bad state.

Let's put the function to wait for the HPD (and the magical 50 ms
after first reset) back in when we're doing an AUX transfer. This
shouldn't actually make things much slower (assuming the panel is on)
because we should immediately poll and see the HPD high. Mostly this
is just an extra i2c transfer to the bridge.

Fixes: f5aa7d46

 ("drm/bridge: parade-ps8640: Provide wait_hpd_asserted() in struct drm_dp_aux")
Tested-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Pin-yen Lin <treapking@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231221135548.1.I10f326a9305d57ad32cee7f8d9c60518c8be20fb@changeid