intelfb: Fix bug in DPLL disable

Reported in Kernel Bugzilla 9006

Fix an obvious bug in DPLL disable.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>