st/drm: add DRM_CONF_XML_OPTIONS

Allow drivers to return the XML that describes the available config
options.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/include/state_tracker/drm_driver.h

@@ -63,6 +63,8 @@ enum drm_conf {
    DRM_CONF_THROTTLE, /* DRM_CONF_INT. */
    /* Can this driver, running on this kernel, import and export dma-buf fds? */
    DRM_CONF_SHARE_FD, /* DRM_CONF_BOOL. */
+   /* XML string describing the available config options. */
+   DRM_CONF_XML_OPTIONS, /* DRM_CONF_POINTER */
    DRM_CONF_MAX
 };