gpu: ion: remove unnecessary function from system heap

ion_system_contig_heap buffers have an sglist, just call
ion_heap_map_user to map it.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>