drm/gma500: Disable PCI device during shutdown

Use managed disablement of PCI devices via pcim_device_enable().
Disables the PCI device and simplifies error rollback in probe
function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210920141051.30988-3-tzimmermann@suse.de