drm/radeon: remove generic rptr/wptr functions (v2)

Fill in asic family specific versions rather than
using the generic version.  This lets us handle asic
specific differences more easily.  In this case, we
disable sw swapping of the rtpr writeback value on
r6xx+ since the hw does it for us.  Fixes bogus
rptr readback on BE systems.

v2: remove missed cpu_to_le32(), add comments

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>