drm/msm: make fb debugfs file available only in KMS case

Don't register the 'fb' debugfs file, if there is no KMS (and so no
framebuffers).

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/561650/


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