drm/msm/dpu: Add a function to retrieve the current CTL status

Add a function that returns whether the requested CTL is active or not:
this will be used in a later commit to fix command mode panel issues.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Link: https://lore.kernel.org/r/20210911163919.47173-1-angelogioacchino.delregno@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>