drm/radeon/kms: workaround invalid AVI infoframe checksum issue

This change was verified to fix both issues with no video I've
investigated. I've also checked checksum calculation with fglrx on:
RV620, HD54xx, HD5450, HD6310, HD6320.

Cc: stable@vger.kernel.org
Signed-off-by: NRafał Miłecki <zajec5@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r600_hdmi.c

@@ -196,6 +196,13 @@ static void r600_hdmi_videoinfoframe(
 	frame[0xD] = (right_bar >> 8);
 
 	r600_hdmi_infoframe_checksum(0x82, 0x02, 0x0D, frame);
+	/* Our header values (type, version, length) should be alright, Intel
+	 * is using the same. Checksum function also seems to be OK, it works
+	 * fine for audio infoframe. However calculated value is always lower
+	 * by 2 in comparison to fglrx. It breaks displaying anything in case
+	 * of TVs that strictly check the checksum. Hack it manually here to
+	 * workaround this issue. */
+	frame[0x0] += 2;
 
 	WREG32(offset+R600_HDMI_VIDEOINFOFRAME_0,
 		frame[0x0] | (frame[0x1] << 8) | (frame[0x2] << 16) | (frame[0x3] << 24));