drm/i915: hw state readout&check support for cpu_transcoder

This allows us to drop a bunch of ugly hacks and finally implement
what

commit cc464b2a17
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Jan 25 16:59:16 2013 -0200

    drm/i915: set TRANSCODER_EDP even earlier

tried to achieve, but that was reverted again in

commit bba2181c49
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Mar 22 10:53:40 2013 +0100

    Revert "drm/i915: set TRANSCODER_EDP even earlier"

Now we should always have a consistent cpu_transcoder in the
pipe_config.

v2: Fix up the code as spotted by Paulo:
- read the register for real
- assign the right pipes
- break out if the hw state doesn't make sense

v3: Shut up gcc.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ddi.c

@@ -1291,9 +1291,13 @@ static bool intel_ddi_compute_config(struct intel_encoder *encoder,
 				     struct intel_crtc_config *pipe_config)
 {
 	int type = encoder->type;
+	int port = intel_ddi_get_encoder_port(encoder);
 
 	WARN(type == INTEL_OUTPUT_UNKNOWN, "compute_config() on unknown output!\n");
 
+	if (port == PORT_A)
+		pipe_config->cpu_transcoder = TRANSCODER_EDP;
+
 	if (type == INTEL_OUTPUT_HDMI)
 		return intel_hdmi_compute_config(encoder, pipe_config);
 	else