drm/radeon: Remove a bunch of useless _iomem casts

Just defining rdev->rmmio properly in the first place should do
the trick. In some cases, the cast were also complete dups as
the original variable was already of the right type.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>