drm/i915/chv: Handle video DIP registers on CHV

The DIP registers are a mess on VLV and CHV. The register block on pipe
A is different than the register block on pipes B and C. In order to
handle that using the pipe offsets, we'd need a new pipe offset per
register, which seems wasteful. So instead just use the _PIPE3() macro
to handle these registers.

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