drm/udl: Inline DPMS code into CRTC enable and disable functions

DPMS functionality is only used by the CRTC's enable and disable
functions. Inline the code. The patch also adds symbolic constants
for the blank register and constants; according to udlfb, which is
a bit more detailed than DRM's udl.

v3:
	* use symbolic constants for blank, according to udlfb driver

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210084905.5570-5-tzimmermann@suse.de