drm/i915: Move intel_atomic_get_shared_dpll_state() to intel_dpll_mgr.c

The function intel_atomic_get_shared_dpll_state() is only called from
intel_dpll_mgr.c and it concerns the same data structures as the other
functions in that file, so move it there and make it static.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483024933-3726-8-git-send-email-ander.conselvan.de.oliveira@intel.com