drm/ast: Set PCI config before accessing I/O registers

Access to I/O registers is required to detect and set up the
device. Enable the rsp PCI config bits before. While at it,
convert the magic number to macro constants.

Enabling the PCI config bits was done after trying to detect
the device. It was probably too late at this point.

v2:
	* use standard 16-bit PCI r/w access (Jingfeng)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-7-tzimmermann@suse.de