msm: mdss: enable fb_split feature for dual-dsi panels

Currently fb_split feature is limited to dual-pipe but single
interface, i.e eDP 2560x1600, panels. Extend use of this feature
to dual-dsi panels as well and by default enable equal split to
support dual-dsi portrait panels, i.e 1600x2560. Default settings
will be over-ridden by split settings from device tree if available.

Change-Id: I38279e1af7dcd1278fa203f51b8b33f82d0848af
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>