drm/radeon: protect against div by 0 in backend setup

Make sure at least one RB is enabled in
r6xx_remap_render_backend() to avoid an division by
zero in some corner cases.

See:
https://bugzilla.redhat.com/show_bug.cgi?id=892233

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/r600.c

@@ -1462,12 +1462,15 @@ u32 r6xx_remap_render_backend(struct radeon_device *rdev,
 			      u32 disabled_rb_mask)
 {
 	u32 rendering_pipe_num, rb_num_width, req_rb_num;
-	u32 pipe_rb_ratio, pipe_rb_remain;
+	u32 pipe_rb_ratio, pipe_rb_remain, tmp;
 	u32 data = 0, mask = 1 << (max_rb_num - 1);
 	unsigned i, j;
 
 	/* mask out the RBs that don't exist on that asic */
-	disabled_rb_mask |= (0xff << max_rb_num) & 0xff;
+	tmp = disabled_rb_mask | ((0xff << max_rb_num) & 0xff);
+	/* make sure at least one RB is available */
+	if ((tmp & 0xff) != 0xff)
+		disabled_rb_mask = tmp;
 
 	rendering_pipe_num = 1 << tiling_pipe_num;
 	req_rb_num = total_max_rb_num - r600_count_pipe_bits(disabled_rb_mask);