drm/i915: correct intel_dp_get_config() function for DevCPT

On DevCPT, the control register for Transcoder DP Sync Polarity is
TRANS_DP_CTL, not DP_CTL.
Without this patch, Many call trace occur on CPT machine with DP monitor.
The call trace is like: *ERROR* mismatch in adjusted_mode.flags(expected X,found X)

v2: use intel-crtc to simple patch, suggested by Daniel.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
[danvet: Extend the encoder->get_config comment to specify that we now
also depend upon intel_encoder->base.crtc being correct. Also bikeshed
s/intel_crtc/crtc/.]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65287
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 9ddbe3b..c8c9b6f 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -141,7 +141,8 @@
 	bool (*get_hw_state)(struct intel_encoder *, enum pipe *pipe);
 	/* Reconstructs the equivalent mode flags for the current hardware
 	 * state. This must be called _after_ display->get_pipe_config has
-	 * pre-filled the pipe config. */
+	 * pre-filled the pipe config. Note that intel_encoder->base.crtc must
+	 * be set correctly before calling this function. */
 	void (*get_config)(struct intel_encoder *,
 			   struct intel_crtc_config *pipe_config);
 	int crtc_mask;