drm/i915/overlay: Use non-atomic mappings for the common case.

The only time where an atomic mapping is required is during
error-capture and there we cannot use the default slot, but need to
specifically use one of the IRQ slots. So separate out the two
conditions and use the atomic mapping only when appropriate.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>