drm/i915: fix up pch pll enabling for pixel multipliers

We have a nice comment saying that the pixel multiplier only sticks
once the vco is on and stable. The only problem is that the enable bit
wasn't set at all. This patch fixes this and so brings the ilk+ pch
pll code in line with the i8xx/i9xx pll code. Or at least improves
matters a lot.

This should fix sdvo on ilk-ivb for low-res modes.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>