drm/msm/dp: limit the maximum lane count for multi-functional dongles

Some dongles which support multi-function return the lane count support
in DPCD to 4. But the link training fails for such dongles since all the
lanes do not support DisplayPort functionality. Limit the maximum lane
count for such dongles to 2.

Change-Id: I19e0fc8d914b4a1f1bedb680772b8f10a38240b1
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>