drm/i915: Apply OCD to data/link m/n register #defines

- PCH_ prefix for pch registers on ibx/cpt/ppt.
- Drop the DP_ from the link defines, redundant.
- Drop the GMCH from the data defines and instead give the special g4x
  registers a consistent _G4X postfix.

v2:
- Realign #defines and use tabs (Paulo).

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