msm: mdss: avoid pref-panel selection for dsi

When trying to boot a dual dsi panel with single dsi configs,
the kernel takes the pref-panel for dsi1. This change avoids the
pref-panel selection when the dsi1 panel string is none.

Change-Id: I433d81a3813f359ed0ba513a3d5412b1c476bccc
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>