drm/radeon/kms: fix handling of tex lookup disable in cs checker on r2xx

drm/radeon/kms: fix handling of tex lookup disable in cs checker on r2xx

commit 43b93fbffc2c080dba2e84df6fce8d7e6c0a2581 upstream.

There are cases when multiple texture units have to be enabled,
but not actually used to sample.  This patch checks to see if
the lookup_disable bit is set and if so, skips the texture check.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=25544

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>