drm/msm: fix handling of cmdstream offset

Userspace hasn't used submit cmds with submit_offset != 0 for a while,
but this starts cropping up again with cmdstream sub-buffer-allocation
in libdrm_freedreno.

Doesn't do much good to increment the buf ptr before assigning it.

Fixes: 78b8e5b8 drm/msm: dump a rd GPUADDR header for all buffers in the command
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>

drivers/gpu/drm/msm/msm_rd.c

@@ -316,10 +316,11 @@ static void snapshot_buf(struct msm_rd_state *rd,
 		uint64_t iova, uint32_t size)
 {
 	struct msm_gem_object *obj = submit->bos[idx].obj;
+	unsigned offset = 0;
 	const char *buf;
 
 	if (iova) {
-		buf += iova - submit->bos[idx].iova;
+		offset = iova - submit->bos[idx].iova;
 	} else {
 		iova = submit->bos[idx].iova;
 		size = obj->base.size;
@@ -340,6 +341,8 @@ static void snapshot_buf(struct msm_rd_state *rd,
 	if (IS_ERR(buf))
 		return;
 
+	buf += offset;
+
 	rd_write_section(rd, RD_BUFFER_CONTENTS, buf, size);
 
 	msm_gem_put_vaddr(&obj->base);