drm/edid: Don't parse garbage as DispID blocks

Currently the code assumes that the entire EDID extesion block
can be taken up by the DispID blocks. That is not true. There
is at least always the DispID checksum, and potentially fill
bytes if the extension block uses the interior fill scheme
to pad out to fill EDID block size.

So let's not parse the checksum or the fill bytes as DispID
blocks by having drm_find_displayid_extension() return the
actual length of the DispID data to the caller.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313162054.16009-7-ville.syrjala@linux.intel.comReviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/drm_edid.c

@@ -3216,6 +3216,7 @@ static u8 *drm_find_displayid_extension(const struct edid *edid,
 					int *length, int *idx)
 {
 	u8 *displayid = drm_find_edid_extension(edid, DISPLAYID_EXT);
+	struct displayid_hdr *base;
 	int ret;
 
 	if (!displayid)
@@ -3228,6 +3229,9 @@ static u8 *drm_find_displayid_extension(const struct edid *edid,
 	if (ret)
 		return NULL;
 
+	base = (struct displayid_hdr *)&displayid[*idx];
+	*length = *idx + sizeof(*base) + base->bytes;
+
 	return displayid;
 }