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/intel_hdcp.c

@@ -57,9 +57,9 @@ static bool hdcp_key_loadable(struct drm_i915_private *dev_priv)
 
 	/* PG1 (power well #1) needs to be enabled */
 	for_each_power_well(dev_priv, power_well) {
-		if (power_well->id == id) {
-			enabled = power_well->ops->is_enabled(dev_priv,
-							      power_well);
+		if (power_well->desc->id == id) {
+			enabled = power_well->desc->ops->is_enabled(dev_priv,
+								    power_well);
 			break;
 		}
 	}