drm: omapdrm: Replace DSS manager state check with omapdrm CRTC state

Instead of conditioning planes update based on the DSS manager hardware
state, use the enabled field newly added to the omap_crtc structure.
This reduces the dependency from the DRM layer to the DSS layer.

The enabled field is a transitory measure, the implementation should use
the CRTC atomic state instead. However, given that CRTCs are currently
not enabled/disabled through their .enable() and .disable() operations
but through a convoluted code paths starting at the associated encoder
operations, there is not clear guarantee that the atomic state always
matches the hardware state. This will be refactored later, at which
point the enabled field will be removed.

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