drm/mgag200: Extract device type and flags in mgag200_pci_probe()

The type and flags values are stored in the PCI ID list. Extract them
in the probe function. Makes the device initialization more readable.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702075642.27834-4-tzimmermann@suse.de