drm/i915: Add functions to get a power well's state/name/domains/mask/refcount

Add functions to get a power well's actual- and cached-enabled state,
name, domain mask and refcount, as a step towards making the low-level
power well internals (i915_power_well_ops/desc structs) hidden.

No functional change.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222165137.1004194-8-imre.deak@intel.com