drm/i915: Expose SAGV state via debugfs

Since SAGV is controlled via unidirectional pcode commands
we have no way to query the current state. So instead let's
expose the last programmed state via debugfs. This way we
can at least know whether SAGV should be enabled or not
(which can be important to know when dealing with underruns/etc.).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230131002127.29305-4-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>