msm: mdss: fixed calculation of pll fractional divider

Pll unlocked due to wrong pll fractional divider calculated.
Pll fraction divider should be reminder of 2^20 after vco
rate divided by reference clock rate.

Change-Id: I9e4c2e3c0631e533d114c3e6acf65b71b9bf00d2
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>