drm/ast: Add helpers for VBIOS mode lookup

drm/ast: Add helpers for VBIOS mode lookup

Mode lines are independent from hardware Gen or TX chip, so hide all
VBIOS mode tables in ast_vbios.c.

Move the look-up code for VBIOS modes from ast_vbios_get_mode_info()
to ast_vbios_find_mode(). The new look-up function respects the
supported-mode flags in struct ast_device. For example, if a device
does not have struct ast_device.support_fullhd set, the helper does
not return a valid mode for 1920x1080. Taking the supported-mode flags
into account allows for making the VBIOS tables the single reference
for validating and setting display modes against hardware capabilities.

v2:
- replace mode switch with look-up table (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-11-tzimmermann@suse.de