disp: msm: sde: set predownscale x_0 if value not provided

Currently, the value for predownscale_x_0 is being set based
on source height and destination height. This value should only
be set if userspace has not set a value for it already or if
default scale is enabled through debugfs.

Change-Id: Icf13ac33ae4a1a40bff90cd639428e9a11f96241
Signed-off-by: Samantha Tran <samtran@codeaurora.org>