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

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2845,10 +2845,10 @@ static int i915_power_domain_info(struct seq_file *m, void *unused)
 		enum intel_display_power_domain power_domain;
 
 		power_well = &power_domains->power_wells[i];
-		seq_printf(m, "%-25s %d\n", power_well->name,
+		seq_printf(m, "%-25s %d\n", power_well->desc->name,
 			   power_well->count);
 
-		for_each_power_domain(power_domain, power_well->domains)
+		for_each_power_domain(power_domain, power_well->desc->domains)
 			seq_printf(m, "  %-23s %d\n",
 				 intel_display_power_domain_str(power_domain),
 				 power_domains->domain_use_count[power_domain]);