drm/i915: extract lpt_enable_clkout_dp from lpt_init_pch_refclk

The next step is to modify lpt_enable_clkout_dp to enable support for
"Sequence to enable CLKOUT_DP" and "Sequence to enable CLKOUT_DP
without spread".

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>