drm/ast: Move widescreen and tx-chip detection into separate helpers

Split ast_detect_chip() into three functions and call them one by
one. The new functions detect the transmitter chip and widescreen
support. This will allow for further refactoring.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn> # AST2400
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-14-tzimmermann@suse.de