drm/omap: use dispc_ops

Change omapdrm to get dispc_ops and use that to call the dispc functions
instead or direct function calls.

The change is very straightforward.

The only problem was in omap_crtc_init() which calls pipe2vbl(crtc), and
at that point of time the crtc->dev link, which is used to get the
dispc_ops, has not been set up yet. This patch makes omap_crtc_init()
skip the call to pipe2vbl() and instead calls
dispc_ops->mgr_get_vsync_irq() directly.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>