drm/radeon: avoid segfault on device open when accel is not working.

When accel is not working on device with virtual address space radeon
segfault because the ib buffer is NULL and trying to map it inside the
virtual address space trigger segfault. This patch only map the ib
buffer if accel is working.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>