disp: msm: dsi: Fix the total number of modes calculation

This change updates the parsing of timing nodes to check
the mode of operation as video or command. In command mode,
for each timing node, num of supported dfps rate is always
one. Accordingly, update the num_dfps_rate to 1 for
command mode.

Change-Id: I5098c7e0d4d2320609d6e10031eaef78c1d8b3c1
Signed-off-by: Lipsa Rout <lrout@codeaurora.org>