drm/udl: Increase the default URB list size to 20

It seems that the current size (4) for the URB list is too small on
some devices, and it resulted in the occasional stalls.  Increase the
default URB list size to 20 for working around it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908095115.23396-7-tiwai@suse.de