drm/i915: Move DP structs to shared location

Move the DP structure to shared location so that it can be used from
within the ddi module.

Changes from Paulo:
- Move less code to intel_drv.h
- Remove #include statement
- Replace a tab with a space in train_set

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>