drm/radeon/kms: add LTE/GTE discard + rv515 two sided stencil register.

This adds some rv350+ register for LTE/GTE discard,
and enables the rv515 two sided stencil register.
It also disables the DEPTHXY_OFFSET register which
can be used to workaround the CS checker.
Moves rs690 to proper place in rs600 and uses correct
table on rs600.

Signed-off-by: Dave Airlie <airlied@redhat.com>