drm/radeon/kms: fix potential segfault in r600_ioctl_wait_idle

radeon_gem_wait_idle_ioctl can apparently get called prior to
the vram page being set up or even if accel if false, so make
sure it's valid before using it.

Should fix:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=597636
https://bugs.freedesktop.org/show_bug.cgi?id=29834

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r600.c

@@ -3528,7 +3528,8 @@ void r600_ioctl_wait_idle(struct radeon_device *rdev, struct radeon_bo *bo)
 	/* r7xx hw bug.  write to HDP_DEBUG1 followed by fb read
 	 * rather than write to HDP_REG_COHERENCY_FLUSH_CNTL
 	 */
-	if ((rdev->family >= CHIP_RV770) && (rdev->family <= CHIP_RV740)) {
+	if ((rdev->family >= CHIP_RV770) && (rdev->family <= CHIP_RV740) &&
+	    rdev->vram_scratch.ptr) {
 		void __iomem *ptr = (void *)rdev->vram_scratch.ptr;
 		u32 tmp;