drm/msm: deal with exhausted vmap space better

Some, but not all, callers of obj->vmap() would check if return
IS_ERR().  So let's actually return an error if vmap() fails.  And fixup
the call-sites that were not handling this properly.

Signed-off-by: Rob Clark <robdclark@gmail.com>
diff --git a/drivers/gpu/drm/msm/msm_ringbuffer.c b/drivers/gpu/drm/msm/msm_ringbuffer.c
index 1f14b90..42f5359 100644
--- a/drivers/gpu/drm/msm/msm_ringbuffer.c
+++ b/drivers/gpu/drm/msm/msm_ringbuffer.c
@@ -40,6 +40,10 @@
 	}
 
 	ring->start = msm_gem_vaddr_locked(ring->bo);
+	if (IS_ERR(ring->start)) {
+		ret = PTR_ERR(ring->start);
+		goto fail;
+	}
 	ring->end   = ring->start + (size / 4);
 	ring->cur   = ring->start;