drm: omapdrm: Check the CRTC software state at enable/disable time

The omapdrm DSS manager enable/disable operations check the DSS manager
state to avoid double enabling/disabling. Check the CRTC software state
instead to decrease the dependency of the DRM layer to the DSS layer.
The dispc_mgr_is_enabled() function then be turned into a static
function, but needs to be moved up in its compilation unit to avoid a
forward declaration.

Add a WARN_ON to catch double enable or disable that should be prevented
by the DRM core and would be a clear sign of a bug. The warning should
eventually be removed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>