backend/drm: add support for suggested output position prop

Virtualised outputs use the "suggested X" and "suggested Y" DRM props to
indicate their relative position on the host WM.

This change adds members `suggested_x` and `suggested_y` to `struct
wlr_output` which contain the value of these props. Also, a
`suggested_position` signal is added so that compositors can be notified
if there is a changed in the suggested position.

backend/drm/properties.c

@@ -25,6 +25,8 @@ static const struct prop_info connector_info[] = {
 	{ "PATH", INDEX(path) },
 	{ "link-status", INDEX(link_status) },
 	{ "vrr_capable", INDEX(vrr_capable) },
+	{ "suggested X", INDEX(suggested_x) },
+	{ "suggested Y", INDEX(suggested_y) },
 #undef INDEX
 };