drm/radeon/kms: properly set the CLK_REF bit for DCE3 devices

If the ss clock is external, the CLK_REF bit needs to be set
in the SetPixelClock parameters.  This should fix DP failures
in the channel equalization loop.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>

drivers/gpu/drm/radeon/atombios_crtc.c

@@ -815,6 +815,8 @@ static void atombios_crtc_program_pll(struct drm_crtc *crtc,
 			args.v3.ucPostDiv = post_div;
 			args.v3.ucPpll = pll_id;
 			args.v3.ucMiscInfo = (pll_id << 2);
+			if (ss_enabled && (ss->type & ATOM_EXTERNAL_SS_MASK))
+				args.v3.ucMiscInfo |= PIXEL_CLOCK_MISC_REF_DIV_SRC;
 			args.v3.ucTransmitterId = encoder_id;
 			args.v3.ucEncoderMode = encoder_mode;
 			break;