drm/ast: Find VBIOS mode from regular display size

The ast driver looks up supplied display modes from an internal list of
display modes supported by the VBIOS.

Do not use the crtc_-prefixed display values from struct drm_display_mode
for looking up the VBIOS mode. The fields contain raw values that the
driver programs to hardware. They are affected by display settings like
double-scan or interlace.

Instead use the regular vdisplay and hdisplay fields for lookup. As the
programmed values can now differ from the values used for lookup, set
struct drm_display_mode.crtc_vdisplay and .crtc_hdisplay from the VBIOS
mode.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-9-tzimmermann@suse.de