drm: re-enable error handling

drm_legacy_ctxbitmap_next() returns idr_alloc() which can return
-ENOMEM, -EINVAL or -ENOSPC none of which are -1 . but the call sites
of drm_legacy_ctxbitmap_next() seem to be assuming that the error case
would be -1 (original return of drm_ctxbitmap_next() prior to 2.6.23
was actually -1). Thus reenable error handling by checking for < 0.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: 62968144 ("drm: convert drm context code to use Linux idr")
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1531571532-22733-1-git-send-email-hofrat@osadl.org