Instead of storing the computed line length in a local variable, store
it in the global lcd_line_length variable to make sure the LCD subsystem
can properly draw content for the display resolution.

This probably wasn't noticed yet because the only board where LCD
support is currently enabled is Seaboard, which runs at a 1366x768
resolution. As it happens this is the maximum resolution supported and
also the default that is used to initialize the framebuffer before the
configuration from DT is available.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>