drm/kms/fb: use slow work mechanism for normal hotplug also.

a) slow work is always used now for any fbcon hotplug, as its not
   a fast task and is more suited to being ran under slow work.

b) attempt to not do any fbdev changes when X is running as we'll
   just mess it up. This hooks set_par to hopefully do the changes
   once X hands control to fbdev.

This also adds the nouveau/intel hotplug support.

Signed-off-by: Dave Airlie <airlied@redhat.com>