drm/i915: Unify power well ID enums

Atm, the power well IDs are defined in separate platform specific enums,
which isn't ideal for the following reasons:
- the IDs are used by helpers like lookup_power_well() in a platform
  independent way
- the always-on power well is used by multiple platforms and so needs
  now separate IDs, although these IDs refer to the same thing

To make things more consistent use a single enum instead of the two
separate ones, listing the IDs per platform (or set of very similar
platforms like all GEN9/10). Replace the separate always-on power
well IDs with a single ID.

While at it also add a note clarifying the distinction between regular
power wells that follow a common programming pattern and custom ones
that are programmed in some other way. The IDs for regular power wells
need to stay fixed, since they also define the request and state HW flag
positions in their corresponding power well control register(s).

v2:
- Add comment about id to req,status bit mapping to the enum. (Rodrigo)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170711204236.5618-1-imre.deak@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>