drm: omapdrm: dss: Allocate the DSS private data structure dynamically

The DSS private data structure is currently stored as a global variable.
While no platform with multiple DSS devices currently exists nor is
planned, this doesn't comply with the kernel device model and should
thus be fixed.

Allocate the DSS private data structure dynamically for each DSS
instance and remove the global variable. All code that need access to
the structure now retrieves it dynamically so we can remove the global
variable.

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