drm/i915: Check lane sharing between pipes B & C using atomic state

Makes that code atomic ready.

v2: Acquire crtc_state for the "other" pipe only when needed. (Daniel)

v3: Really only acquire the other state if necessary. (Daniel)

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