drm/omap: fix: disconnect devices when omapdrm module is removed

At omapdrm probe, we install manager ops and connect omapdss devices. This
needs to be undone when omapdrm module is removed so that omapdss is in a
clean state. This ensures that we can re-insert omapdrm module, or some other
module which uses omapdss(like omapfb/omap_vout).

Currently, omapdrm's remove neither uninstalls manager ops, or disconnects
omapdss devices. We make sure that this is done in pdev_remove.

omapdrm establishes connections for omap_dss_device devices when probed. It
should also be responsible to disconnect the devices. Keeping the devices
connected can prevent the panel driver modules from unloading, it also causes
issues when we try to remove or re-insert omapdrm module.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>