video: Add video_is_active function

Add the helper function video_is_active() to test if one video device
is active.

This function can be used in board code to execute operation
only when the display is probed / really used.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>