drm/i915: extend lpt_enable_clkout_dp

Now it implements 3 different sequences from BSpec and also has
support for ULT.

v2: - Change IS_ULT checks for LPT-LP checks
    - Add check for LPT-LP + with_fdi (Ben)
    - Merge DBUFF0/GEN0 bit definitions since they're the same
      register (Ben)
    - DBUFF0 (1<<0) is Disable, not Enable

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