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>