drm: zynqmp_dp: Add debugfs interface for compliance testing

drm: zynqmp_dp: Add debugfs interface for compliance testing

Add a debugfs interface for exercising the various test modes supported
by the DisplayPort controller. This allows performing compliance
testing, or performing signal integrity measurements on a failing link.
At the moment, we do not support sink-driven link quality testing,
although such support would be fairly easy to add.

Additionally, add some debugfs files for ignoring AUX errors and HPD
events, as this can allow testing with equipment that cannot emulate a
DPRX.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
[Tomi: fixed a few minor formatting issues]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809193600.3360015-9-sean.anderson@linux.dev