drm/armada: fix crtc interlace

We support interlace, but this was broken when we could no longer get
a ref on the vblank interrupt.  Arrange to get the ref on the vblank
interrupt after we've re-enabled vblank, and put it before we disable
the vblank.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>