drm/i915: Remove vblank wait from haswell_write_eld

The pipe is off at that point in time, so a vblank wait is simply a
50ms wait. Caught by Jesse's verbose "make vblank wait timeouts WARN"
patch. We've probably had a few versions of this float around already.

To document assumptions put a pipe assert into the same place. And
also add a posting read.

If we ever decide to update the eld and infoframes while the pipe is
already on (e.g. for fastboot) then there's lots of work to do. So
better properly document all the hidden assumptions.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>