drm/ast: Remove dead else branch in POST code

According to the chip detection in ast_detect_chip(), AST2300 and
later always have a PCI revision of 0x20 or higher. Therefore the
code in ast_set_def_ext_reg() can not use the else branch when
selecing the EXT register values. Remove the dead branch and the
related values.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-5-tzimmermann@suse.de