drm/i915: Preserve ddi_pll_sel when allocating new pipe_config

When the modeset code is reached with a CRTC that only needs a flip, the
code that assigns PLLs is skipped. But since there is still a state swap
for that CRTC, the current PLL assignment needs to be preserved. I
missed the ddi_pll_sel field in the following commit, which causes
warnings in DDI platforms.

commit 4978cc93
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Tue Apr 21 17:13:21 2015 +0300

    drm/i915: Preserve shared DPLL information in new pipe_config

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90410


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