drm/i915: add pipe asserts for the crtc enable sequence

The i9xx modeset sequence is currently pretty fishy, so tight it all
up with some good assert-sprinkling.

We already have good coverage on the disable side, but the enable side
is spotty (since until recently it was wrong).

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