Revert "drm/i915/dp: change aux_ctl reg read to polling read"

This reverts commit 5a9b0c7418448ed3766f61ba0a71d08f259c3181.

The switch from AUX interrupts to pollign was very hand-wavy.
Yes, there have been some situations in CI on a few platforms
where the AUX hardware seemingly forgets to signal the timeout,
but those have been happening after we switched to polling as
well. So I don't think we have any conclusive evidence that
polling actually helps here.

Someone really should root cause the actual problem, and see
if there is a proper workaround we could implemnt (eg. disabling
clock gating/etc.). In the meantime just go back to using the
interrupt for AUX completion.

If the hardware fails to signal the timeout we will just hit
the wait_event_timeout() software timeout instead. I suppose
we could try to tune the software timeout to more closely
match the expected hardware timeout. Might need to use
wait_event_hrtimeout() or something to avoid jiffies
granularity issues...

The AUX polling is also a hinderance towards using poll_timeout_us()
because we have a very long timeout, but would need a fairly short
polling interval to keep AUX transfer reasonably fast. Someone would
need to come up with good numbers in a somewhat scientific way.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251119185310.10428-3-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>

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