drm/msm: only register 'kms' debug file if KMS is used

There is little point in having the empty debugfs file which always
returns -ENODEV. Change this file to be created only if KMS is actually
used.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/561656/


Signed-off-by: Rob Clark <robdclark@chromium.org>