Support reading edid through aux channel if panel is connected to aux
bus. Extend anx7625_aux_dpcd_trans() to implement aux transfer function:

1. panel is populated in devm_of_dp_aux_populate_ep_devices(), so move
   anx7625_parse_dt() after.
2. Use pm runtime autosuspend since aux transfer function is called
   multiple times when reading edid.
3. No-op if aux transfer length is 0.
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Xin Ji <xji@analogixsemi.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220213103437.3363848-3-hsinyi@chromium.org