drm/radeon: fix a bug in the SA code

Aligning offset can make it bigger than tmp->offset
leading to an overrun bug in the following subtraction.

v2: Against initial suspicions this can't happen in mainline,
    so no need to push it into stable.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>