video: mxsfb: Configure the clock after eLCDIF reset

Running stress reboot test on a i.MX6ULL evk board with a
custom LCD can lead to splash screen failure (black screen).

After comparing the eLCDIF initialization with the Linux kernel
driver, it was noticed that the eLCDIF reset is the first operation
that needs to be done.

So do like the eLCDIF Linux driver and move the eLCDIF clock
frequency configuration after the eLCDIF reset and just prior to
setting the RUN bit.

With this change applied, no more black screen issues has been
found during overnight reboot stress tests.

Signed-off-by: Fabio Estevam <festevam@gmail.com>