drm/nv50/kms: use dac loadval from vbios, where it's available

Regression from merging the old nv50/nvd9 code together, and may be
needed to fully fix fdo#64904.

The value is ignored completely by the hardware starting from nva3.
Reported-by: NEmil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -1554,7 +1554,9 @@ nv50_dac_detect(struct drm_encoder *encoder, struct drm_connector *connector)
 {
 	struct nv50_disp *disp = nv50_disp(encoder->dev);
 	int ret, or = nouveau_encoder(encoder)->or;
-	u32 load = 0;
+	u32 load = nouveau_drm(encoder->dev)->vbios.dactestval;
+	if (load == 0)
+		load = 340;
 
 	ret = nv_exec(disp->core, NV50_DISP_DAC_LOAD + or, &load, sizeof(load));
 	if (ret || load != 7)