drm/ast: Remove unused code paths for AST 1180

The ast driver contains code paths for AST 1180 chips. The chip is not
supported and the rsp code has never been tested. Simplify the driver by
removing the AST 1180 code.

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/20200617080340.29584-2-tzimmermann@suse.de