drm/radeon: Fix sideport problems on certain RS690 boards

Some RS690 boards with 64MB of sideport memory show up as
having 128MB sideport + 256MB of UMA.  In this case,
just skip the sideport memory and use UMA.  This fixes
rendering corruption and should improve performance.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=35457



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org