drm/ast: astdp: Inline mode-index calculation

Programming the astdp transmitter chip requires a magic value for
individual modes. Inline the helper for calculating the value into
its only caller (i.e., the encoder's atomic_mode_set).

With further refactoring, the atomic check will be able to detect
invalid modes before attempting to program them.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204133209.403327-3-tzimmermann@suse.de