drm/i915: Pass crtc_state->dpll directly to ->find_dpll()

When calculating clocks, just pass a pointer to crtc_state->dpll
directly to the find_dpll() hook. Back when this was introduced in
commit f47709a9

 ("drm/i915: create pipe_config->dpll for clock
state") there was no staged crtc config or atomic crtc state, so it was
possible to overwrite the current configuration on error. That hasn't
been the case for a while now, so finally make it "disappear".

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458576016-30348-10-git-send-email-ander.conselvan.de.oliveira@intel.com