backend/drm: fix NULL data in handle_drm_event
backend/drm: fix NULL data in handle_drm_event

wl_event_loop_add_fd was called with a NULL data argument, but the
function expects the data argument to be set to the wlr_drm_backend.

Fixes: 053ebe7c278b ("backend/drm: terminate display on drmHandleEvent failure")

swaywm/wlroots,SirCmpwn/wlroots,swaywm/wlroots