drm/msm/dsi-staging: fix minimum fps calculation

Currently in dsi driver the minimum fps supported is being
calculated to zero which is not supported. This change fixes
the logic to properly calculate the min fps during boot up.

Change-Id: Ic79b466c39838264b9017ee9213363658433249d
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>