drm/i915: Parametrize AUX registers

v2: Keep some MISSING_CASE() stuff (Jani)
    s/-1/-PIPE_B/ in the register macro
    Fix typo in patch subject
v3: Use PORT_B registers for invalid ports in g4x_aux_ctl_reg() (Jani)
v4: Reorder patches (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com> (v3)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v3)
Link: http://patchwork.freedesktop.org/patch/msgid/1447266856-30249-4-git-send-email-ville.syrjala@linux.intel.com