staging: drm/omap: call omap_gem_roll() in non-atomic ctx

If fbcon calls us from atomic context, push the work off to the
workqueue to avoid calling into the gem/dmm code in an atomic
context.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>