disp: msm: dsi: Fix num of displays for firmware approach

Currently, in firmware approach, the panel node is null
for secondary display. As a result, the number of display
remains one, in case of dual display also. This change
updates the number of displays.

Change-Id: I4b945b8b94d931e0dedf7ca7b27cb1a288b9d6ab
Signed-off-by: Lipsa Rout <lrout@codeaurora.org>