drm/ast: Embed CRTC and connector in struct ast_private

Only single instances of CRTC and connector are supported per
device. Embed both in ast's structure and remove the individual
memory allocations. DRM's CRTC cleanup helpers replace the rsp.
destroy function in ast.

While at it, also convert to_ast_connector() to a function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730135206.30239-2-tzimmermann@suse.de