drm/mgag200: Move PCI-option setup into model-specific code

Split the PCI code into a single call for each model. G200 and G200SE
each contain a dedicated helper with additional instructions. Noteably,
the G200ER has no code for PCI setup.

In a later patch, the magic numbers should be replaced by descriptive
constants.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112522.5774-4-tzimmermann@suse.de