drm/msm/dp: Read DPCD and sink count in bridge detect()

drm/msm/dp: Read DPCD and sink count in bridge detect()

Instead of relying on the link_ready flag to specify if DP is connected,
read the DPCD bits and get the sink count to accurately detect if DP is
connected.

Signed-off-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Tested-by: Val Packett <val@packett.cool> # x1e80100-dell-latitude-7455
Tested-by: Yongxing Mou <yongxing.mou@oss.qualcomm.com> # Hamoa IOT EVK, QCS8300 Ride
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/727603/
Link: https://lore.kernel.org/r/20260524-hpd-refactor-v6-3-cf3ab488dd7b@oss.qualcomm.com