As documented in the description of the transfer() function of
"struct drm_dp_aux", the transfer() function can be called at any time
regardless of the state of the DP port. Specifically if the kernel has
the DP AUX character device enabled and userspace accesses
"/dev/drm_dp_auxN" directly then the AUX transfer function will be
called regardless of whether a DP device is connected.

For eDP panels we have a special rule where we wait (with a 5 second
timeout) for HPD to go high. This rule was important before all panels
drivers were converted to call wait_hpd_asserted() and actually can be
removed in a future commit.

For external DP devices we never checked for HPD. That means that
trying to access the DP AUX character device (AKA `hexdump -C
/dev/drm_dp_auxN`) would very, very slowly timeout. Specifically on my
system:
  $ time hexdump -C /dev/drm_dp_aux0
  hexdump: /dev/drm_dp_aux0: Connection timed out
  real    0m8.200s
We want access to the drm_dp_auxN character device to fail faster than
8 seconds when no DP cable is plugged in.

Let's add a test to make transfers fail right away if a device isn't
plugged in. Rather than testing the HPD line directly, we have the
dp_display module tell us when AUX transfers should be enabled so we
can handle cases where HPD is signaled out of band like with Type C.

Fixes: c943b494 ("drm/msm/dp: add displayPort driver support")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/583127/
Link: https://lore.kernel.org/r/20240315143621.v2.1.I16aff881c9fe82b5e0fc06ca312da017aa7b5b3e@changeidSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>