drm/i915: Remove erronous WARN in the vlv pipe crc code

It's been in there since forever, and no one cared. Doesn't put a too
good light onto our bug handling and QA efforts really ...

References: https://bugs.freedesktop.org/attachment.cgi?id=90970


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>