drm/i915/dp: Add workaround for spurious AUX timeouts/hotplugs on LTTPR links

To avoid AUX timeouts and subsequent spurious hotplug interrupts, make
sure that the first DPCD access during detection is a read from an LTTPR
register.

Some ADLP DP link configuration at least with multiple LTTPRs expects
the first DPCD access during the LTTPR/DPCD detection after hotplug to
be a read from the LTTPR range starting with
DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV. The side effect of
this read is to put each LTTPR into the LTTPR transparent or LTTPR
non-transparent mode.

The lack of the above read may leave some of the LTTPRs in non-LTTPR
mode, while other LTTPRs in LTTPR transparent or LTTPR non-transparent
mode (for instance LTTPRs after system suspend/resume that kept their
mode from before suspend). Due to the different AUX timeouts the
different modes imply, the DPCD access from a non-LTTPR range will
timeout and lead to an LTTPR generated hotplug towards the source (which
the LTTPR firmware uses to account for buggy TypeC adapters with a long
wake-up delay).

SYSCROS: 72939

v2: Keep DPCD read-out working on non-LTTPR platforms.
v3: Summarize what and why the patch does at the beginning of the commit
    log. (Jani)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220408224629.845887-1-imre.deak@intel.com