drm/i915: Always use adpa_reg

Instead of using ADPA/VLV_ADPA/PCH_ADPA in various parts of
intel_crt code, just use adpa_reg which always contains the
correct value for the platform.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>