gpu: ion: fill in buffer->{dev,size} before mapping new buffers

At least one map_dma() implementation (EXYNOS_CONTIG) assumes the fields
are filled in

Signed-off-by: Greg Hackmann <ghackmann@google.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>