drm/dp_mst: Fix drm RAD print

[Why]
The RAD of sideband message printed today is incorrect.
For RAD stored within MST branch
- If MST branch LCT is 1, it's RAD array is untouched and remained as 0.
- If MST branch LCT is larger than 1, use nibble to store the up facing
port number in cascaded sequence as illustrated below:

u8 RAD[0] = (LCT_2_UFP << 4) | LCT_3_UFP
RAD[1] = (LCT_4_UFP << 4) | LCT_5_UFP
...

In drm_dp_mst_rad_to_str(), it wrongly to use BIT_MASK(4) to fetch the port
number of one nibble.

[How]
Adjust the code by:
- RAD array items are valuable only for LCT >= 1.
- Use 0xF as the mask to replace BIT_MASK(4)

V2:
- Document how RAD is constructed (Imre)

V3:
- Adjust the comment for rad[] so kdoc formats it properly (Lyude)

Fixes: 2f015ec6eab6 ("drm/dp_mst: Add sideband down request tracing + selftests")
Cc: Imre Deak <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Cc: Harry Wentland <[e-mail]>
Cc: Lyude Paul <[e-mail]>
Reviewed-by: Lyude Paul <[e-mail]>
Signed-off-by: Wayne Lin <[e-mail]>
Signed-off-by: Lyude Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/display/drm_dp_mst_topology.c

···
 drm_dp_mst_rad_to_str(const u8 rad[8], u8 lct, char *out, size_t len)
 {
 	int i;
-	u8 unpacked_rad[16];
+	u8 unpacked_rad[16] = {};
 
-	for (i = 0; i < lct; i++) {
+	for (i = 1; i < lct; i++) {
 		if (i % 2)
-			unpacked_rad[i] = rad[i / 2] >> 4;
+			unpacked_rad[i] = rad[(i - 1) / 2] >> 4;
 		else
-			unpacked_rad[i] = rad[i / 2] & BIT_MASK(4);
+			unpacked_rad[i] = rad[(i - 1) / 2] & 0xF;
 	}
 
 	/* TODO: Eventually add something to printk so we can format the rad

include/drm/display/drm_dp_mst_helper.h

···
 	 */
 	struct list_head destroy_next;
 
+	/**
+	 * @rad: Relative Address of the MST branch.
+	 * For &drm_dp_mst_topology_mgr.mst_primary, it's rad[8] are all 0,
+	 * unset and unused. For MST branches connected after mst_primary,
+	 * in each element of rad[] the nibbles are ordered by the most
+	 * signifcant 4 bits first and the least significant 4 bits second.
+	 */
 	u8 rad[8];
 	u8 lct;
 	int num_ports;