drm/i915: sanitize PUNIT register macro definitions

In the upcoming patches we'll need to access the rest of the fields in
the punit power gating register, so prepare for that.

v2:
- add doc reference for the power well subsystem IDs (Jesse)
- remove IDs for non-existant DPIO_RX[23] subsystems (Jesse)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>