drm/i915: Store aux data reg offsets in intel_dp->aux_ch_data_reg[]

Rather than computing on demand, store also the aux data reg
offsets under intel_dp.

v2: Duplicate some code to make things less magic (Jani)
v3: Use PORT_B registers for invalid ports in g4x_aux_data_reg()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1447266856-30249-6-git-send-email-ville.syrjala@linux.intel.com