drm/udl: Replace semaphore with a simple wait queue

UDL driver uses a semaphore for controlling the emptiness of FIFO in a
slightly funky way.  This patch replaces it with a wait queue and
controls the emptiness with the standard wait_event*() macro instead,
which is a more straightforward implementation.

While we are at it, drop the dead code for delayed semaphore down,
too.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220804075826.27036-2-tiwai@suse.de