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

The sdi private data structure is currently stored as a global
variable. While no platform with multiple SDI 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>