drm/radeon/kms: fix regression is handling >2 heads on cedar/caicos

Need to add support for 4 crtcs when setting the possible crtcs
for the encoders.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_encoders.c

@@ -2323,6 +2323,9 @@ radeon_add_atom_encoder(struct drm_device *dev,
 	default:
 		encoder->possible_crtcs = 0x3;
 		break;
+	case 4:
+		encoder->possible_crtcs = 0xf;
+		break;
 	case 6:
 		encoder->possible_crtcs = 0x3f;
 		break;