backend/drm: set drmEventContext version to 2
As per [1] set drmEventContext version to 2, since wlroots does not use the
page_flip_handler2.
[1]:
@@ -1261,7 +1261,7 @@ static void page_flip_handler(int fd, unsigned seq,
int handle_drm_event(int fd, uint32_t mask, void *data) {
drmEventContext event = {
- .version = DRM_EVENT_CONTEXT_VERSION,
+ .version = 2,
.page_flip_handler = page_flip_handler,
};
