msm: mdss: Handle null panel name configuration

LK sends default panel parameter string through
kernel command line argument if auto detection
fails or display is not enabled. DSI driver probe
is failing to parse this panel parameter passed by
LK. With this change, DSI driver will select the
preferred primary panel configured in dtsi if
it parsing fails.

Change-Id: Ief4804ae2cc10abf74317d64fb1b889bd41228d2
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>