drm/i915: Calculate a new pipe_config based on new enabled state

With the current implementation of intel_modeset_affected_pipes(), if a
pipe will be enabled then it is in modeset_pipes. We'll remove that mask
in a follow up patch, but want to preserve this behavior, so just make
that explicit.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>