msm: mdss: calculate software scalar pixel extensions

When there is no data available for pixel extensions from user end,
calculate software pixel extensions with zero initial phase.

Change-Id: I8876b27e4265b39374d5520684028aaaf005d7e8
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>