drm/i915/sdvo: Actually print the reason why the SDVO command failed

It's much easier to figure out why the SDVO encoder refuses to cooperate
if we can see what status we got back.

v2: Zero initialize only the first character, not the whole buffer
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20190410170941.28142-1-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_sdvo.c

@@ -522,6 +522,7 @@ static bool intel_sdvo_read_response(struct intel_sdvo *intel_sdvo,
 #define BUF_LEN 256
 	char buffer[BUF_LEN];
 
+	buffer[0] = '\0';
 
 	/*
 	 * The documentation states that all commands will be
@@ -585,7 +586,8 @@ static bool intel_sdvo_read_response(struct intel_sdvo *intel_sdvo,
 	return true;
 
 log_fail:
-	DRM_DEBUG_KMS("%s: R: ... failed\n", SDVO_NAME(intel_sdvo));
+	DRM_DEBUG_KMS("%s: R: ... failed %s\n",
+		      SDVO_NAME(intel_sdvo), buffer);
 	return false;
 }