drm/gma500: Remove generic DRM drivers in probe function

Gma500 currently removes generic fbdev drivers, but ignores
generic DRM drivers. Use aperture helpers to remove all generic
graphics drivers before loading gma500. Makes gma500 compatible
with simpledrm.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211019080942.24356-1-tzimmermann@suse.de