drm/msm/dp: check sink_count before update is_connected status

Link status is different from display connected status in the case
of something like an Apple dongle where the type-c plug can be
connected, and therefore the link is connected, but no sink is
connected until an HDMI cable is plugged into the dongle.
The sink_count of DPCD of dongle will increase to 1 once an HDMI
cable is plugged into the dongle so that display connected status
will become true. This checking also apply at pm_resume.

Changes in v4:
-- none

Fixes: 94e58e2d06e3 ("drm/msm/dp: reset dp controller only at boot up and pm_resume")
Reported-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Fixes: 8ede2ecc

 ("drm/msm/dp: Add DP compliance tests on Snapdragon Chipsets")
Link: https://lore.kernel.org/r/1619048258-8717-2-git-send-email-khsieh@codeaurora.org
Signed-off-by: Rob Clark <robdclark@chromium.org>