Try disabling tiling.  Add:

Option "ColorTiling" "false"
Option "ColorTiling2D" "false"

to the device section of your xorg.conf, then apply this patch to mesa:

diff --git a/src/gallium/drivers/radeon/r600_texture.c
b/src/gallium/drivers/radeon/r600_texture.c
index e30d933..fe806be 100644
--- a/src/gallium/drivers/radeon/r600_texture.c
+++ b/src/gallium/drivers/radeon/r600_texture.c
@@ -725,6 +725,8 @@ static unsigned r600_choose_tiling(struct
r600_common_screen *rscreen,
 {
        const struct util_format_description *desc =
util_format_description(templ->format);

+       return RADEON_SURF_MODE_LINEAR_ALIGNED;
+
        /* MSAA resources must be 2D tiled. */
        if (templ->nr_samples > 1)
                return RADEON_SURF_MODE_2D;

And try running gears on a bare xserver.