udlfb: introduce a rendering mutex

Rendering calls may be done simultaneously from the workqueue,
dlfb_ops_write, dlfb_ops_ioctl, dlfb_ops_set_par and dlfb_dpy_deferred_io.
The code is robust enough so that it won't crash on concurrent rendering.

However, concurrent rendering may cause display corruption if the same
pixel is simultaneously being rendered. In order to avoid this corruption,
this patch adds a mutex around the rendering calls.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: Bernie Thompson <bernie@plugable.com>
Cc: Ladislav Michl <ladis@linux-mips.org>
Cc: <stable@vger.kernel.org>
[b.zolnierkie: replace "dlfb:" with "uldfb:" in the patch summary]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>