mdss: mdp: reset init phase and phase step to default

reset the init phase and phase step registers to default
when the pixel extension are not calculated in userspace.

Change-Id: I876f1628fb46901a07deb1b97a4ab55f924612b0
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>