drm: omapdrm: Split init and cleanup from probe and remove functions

When merging the omapdrm and omapdss drivers there will be not omapdrm
platform device anymore, and thus no associated probe and remove
functions. To prepare for that, split all the initialization code from
the probe function to make it usable without a platform device.
Similarly, split the cleanup code from the remove function.

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