viafb: simplify lcd size "detection"

Remove all occurences of get_lcd_size_method as only the values
GET_LCD_SIZE_BY_VGA_BIOS and GET_LCD_SIZE_BY_USER_SETTING were used
which had the identical code so there is no need to make things look
more complicated than they actually are.
Just a bit of of cleanup, really no regressions expected.
Signed-off-by: NFlorian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>