drm/i915/dsi: fix CHV dsi encoder hardware state readout on port C

Due to "some hardware limitation" the DPI enable bit in port C control
register does not get set on VLV. As a workaround we check the status in
pipe B conf register instead. The workaround was added in

commit c0beefd2
Author: Gaurav K Singh <gaurav.k.singh@intel.com>
Date:   Tue Dec 9 10:59:20 2014 +0530

    drm/i915: Software workaround for getting the HW status of DSI Port C on BYT

Empirical evidence (on Surface 3 with DSI on port C per VBT) shows that
this is the case also on CHV, so extend the workaround to CHV. We still
have the device ready register check in place, so this should not get
confused with e.g. HDMI on pipe B.

This fixes a number of state checker warnings on CHV DSI port C.

Cc: stable@vger.kernel.org
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460724451-13810-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dsi.c

@@ -725,11 +725,12 @@ static bool intel_dsi_get_hw_state(struct intel_encoder *encoder,
 			BXT_MIPI_PORT_CTRL(port) : MIPI_PORT_CTRL(port);
 		bool enabled = I915_READ(ctrl_reg) & DPI_ENABLE;
 
-		/* Due to some hardware limitations on BYT, MIPI Port C DPI
-		 * Enable bit does not get set. To check whether DSI Port C
-		 * was enabled in BIOS, check the Pipe B enable bit
+		/*
+		 * Due to some hardware limitations on VLV/CHV, the DPI enable
+		 * bit in port C control register does not get set. As a
+		 * workaround, check pipe B conf instead.
 		 */
-		if (IS_VALLEYVIEW(dev) && port == PORT_C)
+		if ((IS_VALLEYVIEW(dev) || IS_CHERRYVIEW(dev)) && port == PORT_C)
 			enabled = I915_READ(PIPECONF(PIPE_B)) & PIPECONF_ENABLE;
 
 		/* Try command mode if video mode not enabled */