drm/i915: enable SDEIER later

On the preinstall stage we should just disable all the interrupts, but
we currently enable all the south display interrupts due to the way we
touch SDEIER at the IRQ handlers (note: they are still masked and our
IRQ handler is disabled). Instead of doing that, let's make the
preinstall stage just disable all the south interrupts, and do the
proper interrupt dance/ordering at the postinstall stage, including an
assert to check if everything is behaving as expected.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>