clk: mdss: re-program PLL registers to power-on-reset value

When the supply to PLL digital domain is turned off,
it can result in certain PLL registers to get corrupted.

Make sure to re-program the PLL registers to the
power-on-reset value before starting to program the PLL again
to ensure that it locks reliably.

Change-Id: I63cac884cf11eae60b187f83654f5922a3342d66
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>