drm/i915: Remove redundant power well IDs

Now that we removed dependence on the power well IDs to determine the
control register and request/status flag offsets the only purpose of
power well IDs is to look up power wells directly bypassing the power
domains framework. However this direct lookup isn't needed for most of
the exisiting power wells and hopefully won't be needed for any new
power wells in the future. To make maintenance of the power well ID enum
easier, don't require a unique ID for each power well, only if it's
necessary. Remove the IDs becoming redundant this way and assign to all
the corresponding power wells a new DISP_PW_ID_NONE ID.

After the previous two patches the IDs don't need to have a fixed value,
so remove the explicit initializers and adjust the enum's code comment
accordingly.

v2:
- Keep required ID assignments for HSW_DISP_PW_GLOBAL and ICL_DISP_PW_2.
  (Paulo)

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806095843.13294-8-imre.deak@intel.com