drm/mxsfb: Don't select DRM_KMS_FB_HELPER

Selecting DRM_FBDEV_EMULATION will include the correct settings for
fbdev emulation. Drivers should not override this.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Stefan Agner <stefan@agner.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210415110040.23525-3-tzimmermann@suse.de