r300g: fixup rs690 tiling stride alignment calculations.

r300g: fixup rs690 tiling stride alignment calculations.

The RS690 memory controller prefers things to be on a different
boundary than the discrete GPUs, we had an attempt to fix this,
but it still failed, this consolidates the stride calculation
into one place and removes the really special case check.

This fixes gnome-shell and 16 piglit tests on my rs690 system.

NOTE: This is a candidate for both the 7.9 and 7.10 branches.

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