drm/radeon/kms/atom: force bpc to 8 for now

Using the bpc (bits per color) specified by the monitor
can cause problems in some cases.  Until we get a better
handle on how to deal with those cases, just use a bpc of 8.

Reported-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>