drm/radeon: Fix 3 regressions - since buffer rework

Commit b4fe9454 introduced 3 bugs,
fix them:

* Use the right command dword for second packet offset in
  RADEON_CNTL_PAINT/BITBLT_MULTI.
* Don't leak memory if drm_buffer_copy_from_user() fails.
* Don't call drm_buffer_unprocessed() unless drm_buffer_alloc() and
  drm_buffer_copy_from_user() have been called successfully first.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Pauli Nieminen <suokkos@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>