drm/radeon: use ALIGN instead of open coding it

Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Corbin Simpson <MostAwesomeDude@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>