r600: fix enabled_rb_mask on eg/cm

r600: fix enabled_rb_mask on eg/cm

For eg/cm, the r600_gb_backend_map will always be 0. This is a bug in
the drm kernel driver, as it just just never fills the information in
(it is now being fixed - the history shows it was being filled in when
the query was brand new but got lost shortly thereafter with backend_map
fixes).
This causes r600_query_hw_prepare_buffer to write the "status bit"
(just the highest bit of the occlusion query result) even for active rbes
(all but the first). This doesn't make much sense, albeit I suppose it's mostly
safe. According to the commit history, it's necessary to set these bits for
inactive rbes since otherwise predication will lock up - presumably the hw just
is waiting for the status bit to appear, which will never happen with inactive
rbes. I'd guess potentially predication could be wrong (due to not waiting for
the actual result if the status bit is already there) if this is set for
active rbes.

Discovered while trying to fix predication lockups on Juniper (needs another
patch).

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