Add the missing sanity check to handle workqueue allocation failures.

Fixes: c8afe684 ("drm/msm: basic KMS driver for snapdragon")
Cc: stable@vger.kernel.org      # 3.12
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/525102/
Link: https://lore.kernel.org/r/20230306100722.28485-8-johan+linaro@kernel.orgSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>