drm/gma500: Drop DRM_GMA3600 config option

With support for the MID-related chips removed, only support for
desktop chips is left in the driver. So just build the complete
driver if DRM_GMA500 has been selected. Anyone who wants to enable
the Poulsbo code would probably also want the Cedarview code.

[Patrik: Fixed conflict due to Oaktrail not being dropped]

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210129095604.32423-4-tzimmermann@suse.de