drm/i915/debugfs: move debug printing to intel_display_power.c

The debugfs should have no special privileges to look into the
implementation guts. Move the actual debug printing of power domains to
intel_display_power.c.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116171434.20516-3-jani.nikula@intel.com