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>