drm/i915/pm: Unstatic power_domain_str

Let us print human-parseable values from the power domain code; upcoming
display code also wants to use it.

This requires moving it out of i915_debugfs.c, as that is only conditionally
compiled.

v2: Move it out of the header.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448034934-11926-1-git-send-email-daniels@collabora.com