drm/gma500: Drop DRM_GMA600 config option

With support for the MID-only Medfield chips removed, simply build
the complete driver if DRM_GMA500 has been selected. Anyone who wants
to enable one of the chips would probably also want the rest.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 837f23bb ("drm/gma500: Drop DRM_GMA3600 config option")
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209112721.3421-1-tzimmermann@suse.de