mdss: ppp: Increase accuracy of scaling factor calculation in ppp

Due to the scaling factor not being accurate enough the pixel
repetition logic for yuv content did not match the scaling logic.
With a more accurate scaling factor we are avoiding problematic
cases with scaling yuv content.

Change-Id: I0b5cb67cff731d02a14c20f658d168f57b350d49
Signed-off-by: Terence Hampson <thampson@codeaurora.org>