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/20220905085744.29637-3-stanislav.lisovskiy@intel.com

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

@@ -59,6 +59,7 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 		&crtc_state->hw.adjusted_mode;
 	bool constant_n = drm_dp_has_quirk(&intel_dp->desc, DP_DPCD_QUIRK_CONSTANT_N);
 	int bpp, slots = -EINVAL;
+	int ret = 0;
 
 	crtc_state->lane_count = limits->max_lane_count;
 	crtc_state->port_clock = limits->max_rate;
@@ -78,10 +79,21 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
 									       crtc_state->lane_count));
 		if (slots == -EDEADLK)
 			return slots;
-		if (slots >= 0)
-			break;
+		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",
 			    slots);