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 4978cc9
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>

drivers/gpu/drm/i915/intel_display.c

@@ -11490,19 +11490,22 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
 	struct intel_crtc_scaler_state scaler_state;
 	struct intel_dpll_hw_state dpll_hw_state;
 	enum intel_dpll_id shared_dpll;
+	uint32_t ddi_pll_sel;
 
 	/* Clear only the intel specific part of the crtc state excluding scalers */
 	tmp_state = crtc_state->base;
 	scaler_state = crtc_state->scaler_state;
 	shared_dpll = crtc_state->shared_dpll;
 	dpll_hw_state = crtc_state->dpll_hw_state;
+	ddi_pll_sel = crtc_state->ddi_pll_sel;
 
 	memset(crtc_state, 0, sizeof *crtc_state);
 
 	crtc_state->base = tmp_state;
 	crtc_state->scaler_state = scaler_state;
 	crtc_state->shared_dpll = shared_dpll;
 	crtc_state->dpll_hw_state = dpll_hw_state;
+	crtc_state->ddi_pll_sel = ddi_pll_sel;
 }
 
 static int