drm/ast: Use per-device logging macros

Converts the ast driver to drm_info(), drm_err() and drm_WARN_*(). No
functional changes are made.

v2:
	* also convert WARN_*() macros

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200617080340.29584-5-tzimmermann@suse.de