drivers/gpu/drm/radeon/r600_blit.c: fix possible NULL pointer derefernce

["Alexander Y. Fomichev" <git.user@gmail.com>]

This patch fix possible NULL pointer dereference when
r600_prepare_blit_copy tries to fill dev_priv->blit_vb->file_priv
without check of dev_priv->blit_vb. dev_priv->blit_vb should be
filled by r600_nomm_get_vb but latest can fail with EAGAIN.
Addresses: https://bugzilla.kernel.org/show_bug.cgi?id=16375

---
 drivers/gpu/drm/radeon/r600_blit.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/drivers/gpu/drm/radeon/r600_blit.c b/drivers/gpu/drm/radeon/r600_blit.c
index f4fb88e..0df4a2b 100644
--- a/drivers/gpu/drm/radeon/r600_blit.c
+++ b/drivers/gpu/drm/radeon/r600_blit.c
@@ -541,6 +541,8 @@ r600_prepare_blit_copy(struct drm_device *dev, struct drm_file *file_priv)
        DRM_DEBUG("\n");

        r600_nomm_get_vb(dev);
+       if (!dev_priv->blit_vb)
+               return;

        dev_priv->blit_vb->file_priv = file_priv;

--
1.7.1.1