drm/i915: extract FDI mPHY functions from lpt_init_pch_refclk

Because lpt_init_pch_refclk implements the "Sequence to enable
CLKOUT_DP for FDI usage and configure PCH FDI I/O", which is very
similar to "Sequence to enable CLKOUT_DP" and "Sequence to enable
CLKOUT_DP without spread". With the extracted functions we can more
easily implement the two missing sequences.

v2: Rebase (WaMPhyProgramming:hsw comment).

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>