atmel_lcdfb: move lcdcon2 register access to compute_hozval

Pass atmel_lcd_info structure to compute_hozval and only do the register
access on SOCs that actually use it.

This will also simplify the removal of the cpu_is macros.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>