drm: omapdrm: dss: Expose DSS data in a dss_device structure

The anoonymous dss structure in dss.c is the top-level component in the
omapdss driver. As such it should store all internal instance-specific
data that is currently stored in global variables. This however requires
both naming the structure to pass it around functions, and accessing it
from various locations in the omapdss driver. While we could implement
get and set functions for every field that needs to be accessed outside
of dss.c, that would introduce overhead and complexity that we could
avoid by exposing the structure to internal components of the omapdss
driver. Do so to prepare for removal of global variables.

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