drm/ast: Split ast_set_vbios_mode_info()

The implementation of ast_set_vbios_mode() converts a DRM display mode
and framebuffer into an adjusted mode and stores information for the
video BIOS to several scratch regsiters.

Here we split the function into individual functions that do the
conversion, set the VBIOS mode information and format information.
This makes it compatible with support for primary planes and atomic
modesetting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-6-tzimmermann@suse.de