drm: omapdrm: Handle CRTC error IRQs directly

Instead of going through a complicated registration mechanism, just
expose the CRTC error IRQ function and call it directly from the main
IRQ handler.

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