drm/udl: Convert to struct drm_simple_display_pipe

Udl has a single display pipeline with a primary plane; perfect for
simple-pipe helpers. Convert it over. The old encoder and CRTC code
becomes unused and obsolete.

Exported formats for the primary plane are RGB565 and XRGB8888, with
the latter being emulated. The 16-bit format is the default and what
is used when communicating with the device.

This patch enables atomic modesetting for udl devices.

v3:
	* remove unused field crtc from struct udl_device
	* set crtc_state->no_vblank at beginning of enable()
v2:
	* move suspend/resume changes into separate patch
	* remove non-atomic code

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210084905.5570-3-tzimmermann@suse.de