drm/i915: add basic Haswell DP link train bits

Previously, the DP register was used for everything. On Haswell, it
was split into DDI_BUF_CTL (which is the new intel_dp->DP register)
and DP_TP_CTL.

The logic behind this patch is based on a patch written by Shobhit
Kumar, but the way the code was written is very different.

Credits-to: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fixup the logic error spotted by Jani Nikula.]
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>