viafb: eliminate modetable dependancy of LCD modesetting

This patch removes the need for knowing the modetable structure
within the LCD modesetting function. The magic 60Hz refresh rate was
already there as always the first entry for a resolution was choosen
based on the ascending refresh rate ordering in the modetable.
For all but one this is at least 60Hz, if only higher frequencies
available we choose those like the code before did. The exception is
OLPC but that resolution has only one frequency so we get the same
behaviour there as well.

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