drm/i915: Fix intel_dp_mst_compute_link_config

We currently always exit that bpp loop because
drm_dp_atomic_find_vcpi_slots doesn't care if we actually
can fit those or not.
I think that wasn't the initial intention here, especially when
we keep trying with lower bpps, we are supposed to keep trying
until we actually find some _working_ configuration, which isn't the
case here.
So added that drm_dp_mst_check here, so that we can make sure
that try all the bpps before we fail.
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101094222.22091-3-stanislav.lisovskiy@intel.com

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -60,6 +60,7 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 	const struct drm_display_mode *adjusted_mode =
 		&crtc_state->hw.adjusted_mode;
 	int bpp, slots = -EINVAL;
+	int ret = 0;
 
 	mst_state = drm_atomic_get_mst_topology_state(state, &intel_dp->mst_mgr);
 	if (IS_ERR(mst_state))
@@ -85,9 +86,20 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 						      connector->port, crtc_state->pbn);
 		if (slots == -EDEADLK)
 			return slots;
-		if (slots >= 0)
+		if (slots >= 0) {
+			ret = drm_dp_mst_atomic_check(state);
+			/*
+			 * If we got slots >= 0 and we can fit those based on check
+			 * then we can exit the loop. Otherwise keep trying.
+			 */
+			if (!ret)
 				break;
 		}
+	}
+
+	/* Despite slots are non-zero, we still failed the atomic check */
+	if (ret && slots >= 0)
+		slots = ret;
 
 	if (slots < 0) {
 		drm_dbg_kms(&i915->drm, "failed finding vcpi slots:%d\n",