drm/omap: do not use BUG_ON(!spin_is_locked(x))

spin_is_locked(x) returns always 0 on uniprocessor, triggering BUG() in
omapdrm.

Change it to use assert_spin_locked() to fix the issue.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>