drm/armada: remove armada_drm_plane_work_cancel() return value

armada_drm_plane_work_cancel()'s returned work structure is never used
or referenced, so it's pointless returning it.  It's also pointless
because the caller doesn't have a clue what kind of work structure it
is.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>