drm/omap: Get from CRTC to display device directly

The CRTC mode set implementation needs to access the omap_dss_device for
the pipeline display. To do so, it iterates over all pipelines to find
the one that contains an encoder corresponding to the CRTC, and request
the display device from the encoder. That's a very complicated dance
when the CRTC has a direct pipeline pointer already, and the pipeline
contains a pointer to the display device.

Replace the convoluted code with direct access.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>