drm/vmwgfx: Refactor module load to not require fifo unless fbdev is loaded

This also fixes a bug where the fence manager was left without irq
enabled when waiting for fences, causing various errors at module
load time

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>