msm: mdss: fix pixel extension validation on decimation

When decimating and if chroma is to be downsampled, hw doesn't decimate
for first step. This needs to be considered as part of the pixel
extension values validation.

Change-Id: I58e61776be4dcc11c105bb46e69c9b486ef1f4f9
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>