drm/ast: Add empty initializer for VBIOS modes

VBIOS mode tables are terminated by an entry with a refresh rate of
0xff. The code is hard to read and fragile to use. Therefore create
an empty entry with AST_VBIOS_MODE_INVALID to terminate each mode list.

Stop at the invalid entry when searching for modes in the tables.
Instead of testing for refresh == 0xff, test with a helper function if
the mode's size and refresh have meaningful values.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-10-tzimmermann@suse.de