drm/ast: Rename AST_IO_VGA_ENABLE_PORT to AST_IO_VGAER

Rename AST_IO_VGA_ENABLE_PORT to AST_IO_VGAER 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/20231017083653.10063-4-tzimmermann@suse.de