drm: Remove unused drm_platform midlayer

Now that the last driver has been converted, the drm_platform midlayer
is unused. Remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sean Paul <seanpaul@chromium.org>