fix picodvi refresh rates (use 60Hz)

This implements video mode timing changes suggested by @WolfWings
in circuitpython issue #10242. Previously, picodvi was set so that
640x480 and 720x400 used refresh rates faster than 60Hz. These
changes lower the refresh rate to 60Hz which should hopefully be
compatible with a wider range of HDMI devices.

Author: samblenny

ports/raspberrypi/common-hal/picodvi/Framebuffer_RP2350.c

@@ -54,15 +54,15 @@
 #define SYNC_V1_H1 (TMDS_CTRL_11 | (TMDS_CTRL_00 << 10) | (TMDS_CTRL_00 << 20))
 
 #define MODE_720_H_SYNC_POLARITY 0
-#define MODE_720_H_FRONT_PORCH   24
-#define MODE_720_H_SYNC_WIDTH    64
-#define MODE_720_H_BACK_PORCH    88
+#define MODE_720_H_FRONT_PORCH   8
+#define MODE_720_H_SYNC_WIDTH    32
+#define MODE_720_H_BACK_PORCH    40
 #define MODE_720_H_ACTIVE_PIXELS 720
 
 #define MODE_720_V_SYNC_POLARITY 0
 #define MODE_720_V_FRONT_PORCH   3
 #define MODE_720_V_SYNC_WIDTH    4
-#define MODE_720_V_BACK_PORCH    13
+#define MODE_720_V_BACK_PORCH    218
 #define MODE_720_V_ACTIVE_LINES  400
 
 #define MODE_640_H_SYNC_POLARITY 0
@@ -74,7 +74,7 @@
 #define MODE_640_V_SYNC_POLARITY 0
 #define MODE_640_V_FRONT_PORCH   10
 #define MODE_640_V_SYNC_WIDTH    2
-#define MODE_640_V_BACK_PORCH    33
+#define MODE_640_V_BACK_PORCH    133
 #define MODE_640_V_ACTIVE_LINES  480
 
 #define MODE_720_V_TOTAL_LINES  ( \