drm/exynos: Zero the dma_addr before decreasing the refcount

drm/exynos: Zero the dma_addr before decreasing the refcount

Otherwise if refcount goes to 0, the buffer is freed, and we do
a use-after-free when setting the dma_addr.

BUG=chrome-os-partner:14832
TEST=compiles

Change-Id: Ie75426b43b6d3519afdab38f19031cf494715ca9
Reviewed-on: https://gerrit.chromium.org/gerrit/34532
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Commit-Ready: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>