drm/virtio: Pass along the format info from .fb_create() to drm_helper_mode_fill_fb_struct()

Plumb the format info from .fb_create() all the way to
drm_helper_mode_fill_fb_struct() to avoid the redundant
lookup.

Cc: David Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Chia-I Wu <olvaffe@gmail.com>
Cc: virtualization@lists.linux.dev
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250701090722.13645-18-ville.syrjala@linux.intel.com