disp: msm: sde: add default calculations and settings for pre-downscale

This change introduces pre-downscaling values to the path where
these values are not provided by userspace. Currently, pre-downscaling
is only allowed by a factor of 2.2 in the x direction. With this
change pre-downscaling will support >2.2 up to 4 in the x direction.

Change-Id: I04d1b07243a5973e9338ea2a212280985b31b6a3
Signed-off-by: Samantha Tran <samtran@codeaurora.org>