drm/i915: Don't use pipe_offset stuff for DPLL registers

These are just single registers so wasting space for the pipe offsets
seems a bit pointless. So just use the _PIPE3() macro instead.

Also rewrite the _PIPE3() macro to be more obvious, and protect the
arguments properly.

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