disp: msm: dsi: update panel mode parsing message for POMS

It should not be an error that panel mode isn't specified
in timing node, so add this change to lower the log level
from error to info.

Change-Id: I49bd1fec1c09697d9829a8e0767dfa3cf2cff512
Signed-off-by: Lei Chen <chenlei@codeaurora.org>