drm/i915/dp: Add device specific quirk to limit eDP rate to HBR2

Some ICL/TGL platforms with combo PHY ports suffer from signal integrity
issues at HBR3. While certain systems include a Parade PS8461 mux to
mitigate this, its presence cannot be reliably detected. Furthermore,
broken or missing VBT entries make it unsafe to rely on VBT for enforcing
link rate limits.

To address this introduce a device specific quirk to cap the eDP link rate
to HBR2 (540000 kHz). This will override any higher advertised rates from
the sink or DPCD for specific devices.

Currently, the quirk is added for Dell XPS 13 7390 2-in-1 which is reported
in gitlab issue #5969 [1].

[1] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/5969

v2: Align the quirk with the intended quirk name and refactor the
condition to use min(). (Jani)
v3: Use condition `rate > 540000`. Drop extra parentheses. (Ville)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/5969
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://lore.kernel.org/r/20250710052041.1238567-3-ankit.k.nautiyal@intel.com

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