Trebuchet: force close in dock (Trebuchet) with raised default page count

Use the same ui_homescreen_default_screen's algorithm to fix overflow default values

Change-Id: I2f529dc902fd736c2e36ff92cdf001bd23518236
JIRA: CYAN-757
Issue: https://jira.cyanogenmod.org/browse/CYAN-757
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>