drm/i915: Add crtc states before calling compute_config()

The function intel_modeset_compute_config() needs to eventually become
part of atomic_check(). At that point, all the affected crtcs need to be
in the atomic state with the new values. So move the logic of adding
crtc states out of that function.

v2: Set crtc_state->enable in all cases. (Ander)

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>