drm: Warn when leaking flip events on close

Warn when there are events on the file_priv->event_list just before
file_priv gets freed. This can occur if the driver doesn't clean up
pending page flip events in ->preclose().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

diff --cc drivers/gpu/drm/drm_fops.c
Simple merge