This allows the use of more than 3 ports/pipes/whatever without tricks,
even if the register offsets are not evenly spaced.

There's the risk of out of bounds access if we're not careful; currently
that would "just" lead to the wrong register offset being used. It might
be possible to add build bug ons for build time constant indexing.

We already have ports defined up to E, not sure if we might have bugs
related to them and the current _PORT3() macro.

   text	   data	    bss	    dec	    hex	filename
1239868	  46199	   4096	1290163	 13afb3	drivers/gpu/drm/i915/i915.ko
1238828	  46199	   4096	1289123	 13aba3	drivers/gpu/drm/i915/i915.ko

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: NDaniel Vetter <daniel@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485532626-20923-1-git-send-email-jani.nikula@intel.com