drm/i915: remove SDV support from lpt_pch_init_refclk

The machines that fall in the "is_sdv" case are some very early
pre-production steppings. This patch may break VGA output after
suspend/resume on these machines.

Even the documentation for the is_sdv cases was removed from BSpec.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>