drm/radeon: map registers at load time

Now that the radeon driver has suspend/resume functions, it needs to map its
registers at load time or it will likely crash if a suspend operation occurs
before the driver has been initialized.

This patch moves the register mapping code from firstopen to load and makes
the mapping into a _DRM_DRIVER one so that the core won't remove it at
lastclose time.

Fixes (at least partially) kernel bz #11891.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>