drm/i915: vlv_prepare_pll is only needed in case of non DSI interfaces

For MIPI, DSI PLL is configured separately in vlv_configure_dsi_pll
during the DSI enable sequence

Causing WARN dump otherwise in dpio_reads

v2: Add IS_CHERRYVIEW check as suggested by Ville
Signed-off-by: NShobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -4564,7 +4564,10 @@ static void valleyview_crtc_enable(struct drm_crtc *crtc)
 	if (intel_crtc->active)
 		return;
 
-	vlv_prepare_pll(intel_crtc);
+	is_dsi = intel_pipe_has_type(crtc, INTEL_OUTPUT_DSI);
+
+	if (!is_dsi && !IS_CHERRYVIEW(dev))
+		vlv_prepare_pll(intel_crtc);
 
 	/* Set up the display plane register */
 	dspcntr = DISPPLANE_GAMMA_ENABLE;
@@ -4598,8 +4601,6 @@ static void valleyview_crtc_enable(struct drm_crtc *crtc)
 		if (encoder->pre_pll_enable)
 			encoder->pre_pll_enable(encoder);
 
-	is_dsi = intel_pipe_has_type(crtc, INTEL_OUTPUT_DSI);
-
 	if (!is_dsi) {
 		if (IS_CHERRYVIEW(dev))
 			chv_enable_pll(intel_crtc);