drm/radeon: make ring rptr and wptr register offsets variable

Every ring seems to have the concept of read and
write pointers. Make the register offset variable
so we can use the functions for different types of rings.

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