drm/i915: always disable irqs in intel_pipe_update_start

This can only fail because of a bug in the code.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Squash in follow-up to also remove start_vbl_count from
intel_crtc->atomic and put it into the intel_crtc directly - it's not
precomputed state.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>