drm/i915: Rename intel_shared_dpll->mode_set() to prepare()

The hook is called from intel_prepare_shared_dpll(). The name doesn't
make sense after all the changes to modeset code. So just call it
prepare.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483024933-3726-5-git-send-email-ander.conselvan.de.oliveira@intel.com