drm/radeon: Disable uncacheable CPU mappings of GTT with RV6xx

They reportedly cause random GPU hangs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91268



Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>