viafb: reset correct PLL

Looks like we did reset the PLL of the (whatever) engine instead of
the PLL of the secondary display (IGA2, LCDCK). This patch fixes it.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>