drm: omapdrm: venc: Allocate the venc private data structure dynamically

The venc private data structure is currently stored as a global
variable. While no platform with multiple VENC encoders currently exists
nor is planned, this doesn't comply with the kernel device model and
should thus be fixed.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>