drm/i915: Don't change the blank/sync width when calculating scaled modes

Also, use the border instead of border minus one.

At the same time, make sure the horizontal border and hsync are even for
the LVDS that works in dual-channel mode. So both horizontal border and hsync
start are also changed to be even, even for the LVDS in single-channel mode.

https://bugs.freedesktop.org/show_bug.cgi?id=20951

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>