msm: mdss: setup dsi control index before using it

The ndx variable of dsi control structure needs to be configured
correctly in the early stage of probe. Otherwise, subsequent function
could dereference to a wrong value.

Change-Id: Icdcdf6713271af25c5135702d8479f5213909560
Signed-off-by: Vinu Deokaran <vinud@codeaurora.org>