drm/i915/dp: Read DPRX for all long HPD pulses

The TBT DP tunnel BW request logic in the Thunderbolt Connection Manager
depends on the GFX driver reading out the sink's DPRX capabilities in
response to a long HPD pulse. Since in i915 this read-out can be blocked
by another connector's/encoder's hotplug event handling (which is
serialized by drm_mode_config::connection_mutex), do a dummy DPRX read-out
in the encoder's HPD pulse handler (which is not blocked by other
encoders).

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240220211841.448846-21-imre.deak@intel.com