drm/i915: Use crtc state in intel_modeset_pipe_config

Grabbing crtc state from atomic state is a lot more involved,
and make sure connectors are added before calling this function.

Move check_digital_port_conflicts to intel_modeset_checks,
it's only useful to check it on a modeset.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>