drm/i915: allow PCH PWM override on IVB

On IVB, there are two sets of panel backlight regs: one in the CPU and
one in the PCH.  The CPU ones aren't generally used, so on IVB make sure
we allow the PCH regs to actually control the backlight.

v2: remove unused pwm variable (Daniel)
    move to init_hw function so we override on resume too

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