drm/radeon: Fix eDP for single-display iMac11,2

commit 05eacc1 upstream.

Apple iMac11,2 (mid 2010) also with Radeon HD-4670 that has the same
issue as iMac10,1 (late 2009) where the internal eDP panel stays dark on
driver load.  This patch treats iMac11,2 the same as iMac10,1,
so the eDP panel stays active.

Additional steps:
Kernel boot parameter radeon.nomodeset=0 required to keep the eDP
panel active.

This patch is an extension of
commit 564d8a2 ("drm/radeon: Fix eDP for single-display iMac10,1 (v2)")
Link: https://lore.kernel.org/all/lsq.1507553064.833262317@decadent.org.uk/
Signed-off-by: Mark Hawrylak <mark.hawrylak@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -2188,11 +2188,12 @@ int radeon_atom_pick_dig_encoder(struct drm_encoder *encoder, int fe_idx)
 
 	/*
 	 * On DCE32 any encoder can drive any block so usually just use crtc id,
-	 * but Apple thinks different at least on iMac10,1, so there use linkb,
+	 * but Apple thinks different at least on iMac10,1 and iMac11,2, so there use linkb,
 	 * otherwise the internal eDP panel will stay dark.
 	 */
 	if (ASIC_IS_DCE32(rdev)) {
-		if (dmi_match(DMI_PRODUCT_NAME, "iMac10,1"))
+		if (dmi_match(DMI_PRODUCT_NAME, "iMac10,1") ||
+		    dmi_match(DMI_PRODUCT_NAME, "iMac11,2"))
 			enc_idx = (dig->linkb) ? 1 : 0;
 		else
 			enc_idx = radeon_crtc->crtc_id;