drm/radeon/kms: Don't RMW CP_RB_CNTL

Immediate readback seems faulty on some chips.  I
suspect it takes a while to get through the fifo
to the actual register backbone.  There's no need
to read it back, so, just write the driver's copy
of the register's value directly.

Should fix bug 24535 and possibly 24218

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>