Revert "atmel_lcdfb: Adjust HFP calculation so it matches the manual."

This reverts commit 5d910426.

Nicolas Ferre <nicolas.ferre@atmel.com> wrote:
"Unfortunately this is not true for all the SoC that embed the
atmel_lcdfb... So I may need to rework this patch but it is certainly
not applicable in the current form."

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>