drm/ast: Rename AST_IO_DAC_INDEX_READ to AST_IO_VGADRR

Rename AST_IO_DAC_INDEX_READ to AST_IO_VGADRR to align naming
in the driver with documentation. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231017083653.10063-6-tzimmermann@suse.de