drm/ast: astdp: Clean up EDID reading

Simplify ast_astdp_read_edid(). Rename register constants. Drop
unnecessary error handling. On success, the helper returns 0; an
error code otherwise.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240717143319.104012-6-tzimmermann@suse.de