drm/vc4: Fix refcounting of runtime PM get if it errors out.

We were returning without decrementing if the error happened, meaning
that at the next submit we wouldn't try to bring up the power domain.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170417162603.12726-1-eric@anholt.net


Reviewed-by: Sean Paul <seanpaul@chromium.org>
(cherry picked from commit 925d05e1)