drm/ast: astdp: Store mode index in connector state

Look up the mode index for the astdp transmitter ship in the encoder's
atomic check and report an error if the display mode is not supported.

The lookup uses the DRM display mode instead of the driver's internal
VBIOS mode. Both are equivalent. The modesetting code later reads
the calculated index from the connector state to avoid recalculating it.

v2:
- fix typo in commit message (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204133209.403327-4-tzimmermann@suse.de