drm/ast: Remove little-endianism from I/O helpers

Replace one call to ast_io_write16() with two calls to ast_io_write8()
in ast_set_index_reg(). The combined 16-bit-wide write of an index
register and the corresponding data register only works on little-
endian systems. Write both registers independent from each other.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230221155745.27484-2-tzimmermann@suse.de