drm/i915: Pre-compute pipe enabled state

Add 'new_enabled' to intel_crtc and precompute it alongside new_encoder
and new_crtc. This will allow making decisions about shared resources
that are affected by the set of active pipes, before we've clobbered
anything for real.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>