CHROMIUM: drm/exynos: fb: Don't return errors when unmapping fb gem objects

CHROMIUM: drm/exynos: fb: Don't return errors when unmapping fb gem objects

Silently accept and ignore a NULL buffer or a buffer which has not yet
been mapped (i.e., its dma_addr is unset).

These aren't really errors (and I don't see how they are even possible).
Simplifying this lets the unmap function just return void.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
BUG=chromium-os:37097
TEST=builds clean; sanity check on device

Change-Id: Ib1fa50152b0444fbafc05ac8b712d69ffda962c5
Reviewed-on: https://gerrit.chromium.org/gerrit/40446
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>