drm/omap: remove warn from debugfs

Patch dfe96ddc (omapdrm: simplify locking in
the fb debugfs file) removed taking locks when using omapdrm's debugfs
to dump fb objects.

However, in omap_gem_describe we give a WARN is the lock has not been
taken, so that WARN is now seen every time omapdrm debugfs is used.

So, presuming the removal of locks is ok, we can also remove the WARN.

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