drm/msm/dp: Remove error message when downstream port not connected

Plugging in an Apple dongle without the HDMI cable attached prints out
an error message in the kernel logs when nothing is actually wrong.

no downstream ports connected

This is because the downstream port for the HDMI connector is not
connected, so the Apple dongle reports that as a zero sink count device.

Cc: Vinod Polimera <[e-mail]>
Cc: Kuogee Hsieh <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>
Patchwork: https://patchwork.freedesktop.org/patch/556068/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Baryshkov <[e-mail]>

drivers/gpu/drm/msm/dp/dp_panel.c

···
 	if (drm_dp_is_branch(dp_panel->dpcd)) {
 		count = drm_dp_read_sink_count(panel->aux);
 		if (!count) {
-			DRM_ERROR("no downstream ports connected\n");
 			panel->link->sink_count = 0;
 			return -ENOTCONN;
 		}