drm/ast: dp501: Inline ast_dp501_connector_init()

Inline ast_dp501_connector_init() into its only caller. The helper
currently only does half of the connector-init work and is trivial
enough to be inlined.

Also set the local variables for encoder and connector as late as
possible, so that the compiler warns if we use them before having
initialized the instance.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911115347.899148-6-tzimmermann@suse.de