radeon: Unmap vram pages when reclocking

Touching vram while the card is reclocking can lead to lockups. Unmap
any pages that could be touched by the CPU and block any accesses to
vram until the reclocking is complete.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>