drm/omap: fix DMM driver (un)registration

At the moment the DMM driver is never unregistered, even if it's
registered in the omapdrm module's init function. This means we'll get
errors when reloading the omapdrm module.

Fix this by unregistering the DMM driver properly, and also change the
module init to fail if DMM driver cannot be registered, simplifying the
unregister path as we don't need to keep the state whether we registered
the DMM driver or not.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>