backend/session: fix KMS device filtering
backend/session: fix KMS device filtering

As explained in [1], user-space should perform a drmModeGetResources
call to figure out whether a device supports KMS.

[1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/127

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots