drm/ast: astdp: Only test HDP state in ast_astdp_is_connected()

The overall control flow of the driver ensures that it never reads
EDID or sets display state on unconnected outputs. Therefore remove
all tests for Hot Plug Detection from these helpers. Also rename
the register constants.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240717143319.104012-4-tzimmermann@suse.de