drm/i915: add a debugfs entry for power domain info

Add a debugfs entry showing the use-count for all power domains of each
power well.

v3: address comments from Paulo:
- simplify power_domain_str() by using a switch table
- move power_well::domain_count to power_domains
- WARN_ON decrementing a 0 refcount

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 0f7bc14..49dfac9 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -971,6 +971,9 @@
 	int power_well_count;
 
 	struct mutex lock;
+#if IS_ENABLED(CONFIG_DEBUG_FS)
+	int domain_use_count[POWER_DOMAIN_NUM];
+#endif
 	struct i915_power_well *power_wells;
 };