Revert "drm/i915: allow PCH PWM override on IVB"

This reverts commit f82cfb6b.

This breaks the backlight controls on my IVB asus zenbook with an eDP
panel.

I guess the right fix would be to read this bit and use either the pch
or the cpu register to frob the backlight values. But that is stuff
for -next.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>