While the details of getting a shared dpll are wrapped by
intel_get_shared_dpll(), the release was still hand rolled into the
modeset code. Fix that by creating an entry point for releasing the
pll and move that code there.

v2: Take old_dpll from crtc->state instead of crtc_state. (CI)
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483024933-3726-2-git-send-email-ander.conselvan.de.oliveira@intel.com