drm/cirrus: Remove obsolete header file

The cirrus driver's header file is left over from a recent rewrite.
Remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20191017113427.2167-1-tzimmermann@suse.de


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>