Just store function pointers that give us the correct register offsets
instead of storing the register offsets themselves. Slightly less
efficient perhaps but saves a few bytes and better matches how we do
things elsewhere.

v2: Keep a local array of data registers (Chris)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222181036.15251-3-ville.syrjala@linux.intel.com