drm/msm/dp: use drm_bridge_hpd_notify() to report HPD status changes

Currently DP driver use drm_helper_hpd_irq_event(), bypassing drm bridge
framework, to report HPD status changes to user space frame work.
Replace it with drm_bridge_hpd_notify() since DP driver is part of drm
bridge.

Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>