drm/i915: vlv: fix cdclk setting during modeset while suspended

Currently after doing DPMS-OFF on all outputs CDCLK won't be set to its
minimum value as it should. A subsequent modeset to turn off all outputs
will thus run with all power domains disabled, and notice that it needs
to change CDCLK to its minimum value. Since the power domains are
disabled this will emit a register-access-while-suspended WARN and fail
to set the minimum freq.

The proper solution for this is to set the minimum frequency during
DPMS-OFF. That needs a bigger rework that would take into account the
user DPMS setting too during the calculation of the new modesetting
configuration. Until that's done this stop-gap solution gets the PIPE-A
power domain during setting the CDCLK; this domain covers the HW blocks
needed for this.

Idea to use PIPE-A domain from Ville.

Testcase: igt/pm_rpm
Reference: https://bugs.freedesktop.org/show_bug.cgi?id=82939
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -4939,10 +4939,23 @@ static void valleyview_modeset_global_resources(struct drm_device *dev)
 	int req_cdclk = valleyview_calc_cdclk(dev_priv, max_pixclk);
 
 	if (req_cdclk != dev_priv->vlv_cdclk_freq) {
+		/*
+		 * FIXME: We can end up here with all power domains off, yet
+		 * with a CDCLK frequency other than the minimum. To account
+		 * for this take the PIPE-A power domain, which covers the HW
+		 * blocks needed for the following programming. This can be
+		 * removed once it's guaranteed that we get here either with
+		 * the minimum CDCLK set, or the required power domains
+		 * enabled.
+		 */
+		intel_display_power_get(dev_priv, POWER_DOMAIN_PIPE_A);
+
 		if (IS_CHERRYVIEW(dev))
 			cherryview_set_cdclk(dev, req_cdclk);
 		else
 			valleyview_set_cdclk(dev, req_cdclk);
+
+		intel_display_power_put(dev_priv, POWER_DOMAIN_PIPE_A);
 	}
 }