hwc: memset dropArray when changing refresh rate

memset dropArray and set dropCount to 0 before proceeding
to identify if refresh rate needs to be changed. Not doing
this will result in dropCount containing junk value which
will impact mdpCount. This prevents us in changing refresh
rate in cases where it could be changed.

Change-Id: I4081062157620ca0dc0b4a6fed0689b2869efb91
1 file changed