drm: rip out drm_platform_exit

This very much looks like a remnant of the old legady ums shadow
attach days. Now with the last users gone we can rip it out since
we won't ever support an ums drm driver again.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>