drm/msm/dp: Use drm_dp_read_sink_count() helper

Use the common function drm_dp_read_sink_count() instead of open-coding
it. This shrinks the kernel text a tiny bit.

Cc: Vinod Polimera <quic_vpolimer@quicinc.com>
Cc: Kuogee Hsieh <quic_khsieh@quicinc.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/554983/
Link: https://lore.kernel.org/r/20230829184735.2841739-3-swboyd@chromium.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>