drm/udl: Simplify modesetting in CRTC's enable function

Inline a modesetting helper in the CRTC's enable function. Build the
command set directly in the USB URB's buffer and drop an intermediate
buffer. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-9-tzimmermann@suse.de