drm/i915/sdvo: Add helpers to get the cmd/status string

Add sdvo_cmd_name() and sdvo_cmd_status() helpers to avoid bothering
the callers with the implementation details of the storage for these
strings.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190619180312.31817-6-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>