drm/i915: Constify power well descriptors

drm/i915: Constify power well descriptors

It makes sense to keep unchanging data const. Extract such fields from
the i915_power_well struct into a new i915_power_well_desc struct that
we initialize during compile time. For the rest of the dynamic
fields allocate an array of i915_power_well objects in i915 dev_priv,
and link to each of these objects their corresponding
i915_power_well_desc object.

v2:
- Fix checkpatch warnings about missing param name in fn declaration and
  lines over 80 chars. (Paulo)
- Move check for unique IDs to __set_power_wells().

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>
[Fixed checkpatch warn in __set_power_wells()]
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806095843.13294-5-imre.deak@intel.com