drm/kms/radeon: pick digitial encoders smarter. (v3)

booting a Lenovo W500 with LVDS + DP outputs showed up a TODO we had
on our list, to pick a correct digital encoder block. The LVTMA
encoder requires the second digital encoder, all others can use any
encoder at all.

This fixes the digital encoder selection logic to enable LVDS/DP combos
to work okay.

V2: fix silly addition of connector dig_block and cleanup the other
places in the code that pick the encoder.

V3: rename to dig_encoder and clean up further - also fix
the picking algorithm.

tested on Lenovo W500 + desktop 3650 cards.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/radeon_mode.h b/drivers/gpu/drm/radeon/radeon_mode.h
index 96b851f..e81b2ae 100644
--- a/drivers/gpu/drm/radeon/radeon_mode.h
+++ b/drivers/gpu/drm/radeon/radeon_mode.h
@@ -299,7 +299,7 @@
 struct radeon_encoder_atom_dig {
 	/* atom dig */
 	bool coherent_mode;
-	int dig_block;
+	int dig_encoder; /* -1 disabled, 0 DIGA, 1 DIGB */
 	/* atom lvds */
 	uint32_t lvds_misc;
 	uint16_t panel_pwr_delay;