Ensure driver has been initialized (dev_private != NULL) before installing

Ensure driver has been initialized (dev_private != NULL) before installing
    irq handler in DRM(irq_install). Modify all drivers to ensure irq
    handler is removed before cleanup and cleanup is called at takedown.
    Remove unused buffer private struct fields in i810, i830. Check for
    lock on init/cleanup in all drivers except i810/i830. The current DDX
    for i810 and i830 doesn't hold the lock on kernel init (FIXME?).