drm/i915: Don't use encoder->new_crtc in compute_baseline_pipe_bpp()

Move towards atomic by using the legacy modeset's drm_atomic_state
instead.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Keep the if (!connector) continue; separate so that it's
easier to eventually extract a for_each_connector_in_state iterator.
And because of the upcast it's also safer.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>