msm: mdss: Correct phase step calculations for YUV format

In the current implementation, phase step calculation for chroma plane
is not considering chroma subsample and is reduced to half. This is
incorrect and needs to be reduced to half only when chroma sub sampling
is present. This change corrects the phase step calculations for the
chroma planes.

Change-Id: Ia909f909f699882b863efb17b5dded77e6fdb1b1
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>