drm/cma-helper: Fix drm_gem_cma_free_object()

The logic for freeing an imported buffer with a virtual address is
broken. It will free the buffer instead of unmapping the dma buf.
Fix by reversing the if ladder and first check if the buffer is imported.

Fixes: b9068cde

 ("drm/cma-helper: Add DRM_GEM_CMA_VMAP_DRIVER_OPS")
Cc: stable@vger.kernel.org
Reported-by: "Li, Tingqian" <tingqian.li@intel.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426124753.53722-1-noralf@tronnes.org