soc: qcom: pmic_glink_altmode: Fix SVID=DP && unconnected edge case

soc: qcom: pmic_glink_altmode: Fix SVID=DP && unconnected edge case

The commit referenced in Fixes started evaluating the value of
alt_port->mux_ctrl before checking the active SVID. This led to
drm_aux_hpd_bridge_notify() no longer being called for the 'DP unplug'
case.

Perhaps somewhat interestingly, the firmware sends a notification with
SVID=DP, mux_ctrl=MUX_CTRL_STATE_NO_CONN and pin_assignment=0 on
unplug. 'pin_assignment' was previously interpreted as a bitfield
excerpt from the second byte of the DP pg_altmode payload (and stored
as an u8).

That value is used in pmic_glink_altmode_sc8280xp_notify(), decremented
by 1 (DPAM_HPD_A). Previously, this would result in an u8 underflow
that would rollover to 0xff (which prior to the Fixes patch would have
caused a pmic_glink_altmode_safe() and 'disconnected' bridge
notification). That check was removed, without a replacement.

Resolve this issue by making sure the SID=DP && mux_ctrl=NO_CONN combo
once again results in a HPD bridge notification.

Fixes: 0539c5a6fdef ("soc: qcom: pmic_glink_altmode: Consume TBT3/USB4 mode notifications")
Reported-by: Abel Vesa <abel.vesa@oss.qualcomm.com>
Tested-by: Abel Vesa <abel.vesa@oss.qualcomm.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260306-topic-pgaltmode_fixup-v1-1-ec154b2d8e89@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>