drm/mgag200: Use pcim_enable_device()

Using the managed function simplifies the error handling. After
unloading the driver, the PCI device should now get disabled as
well.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-4-tzimmermann@suse.de