libcamera: Replace toString with operator<<() for format classes

Now that format classes implement the stream formatting operator<<(),
use it instead of the toString() function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

test/camera-sensor.cpp

@@ -99,7 +99,7 @@ protected:
 		    format.size != Size(4096, 2160)) {
 			cerr << "Failed to get a suitable format, expected 4096x2160-0x"
 			     << utils::hex(MEDIA_BUS_FMT_SBGGR10_1X10)
-			     << ", got " << format.toString() << endl;
+			     << ", got " << format << endl;
 			return TestFail;
 		}