Revert "drm/i915: assume all GM45 Acer laptops use inverted backlight PWM"

My Acer 8510TZ stops displaying anything when X starts with Linus' current
tree. I bisected it down to commit ee1452d7.

This patch reverts commit ee1452d7.

After the revert, everything works as before.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Reported-by: Dylan Borg <borgdylan@hotmail.com> (for a Acer Extensa 5635Z)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>