drm/amd/display: Don't set mode_changed=false if the stream was removed

[Why]
When switching from vt to desktop with EDID emulation we can receive
an atomic commit such that we have a crtc where mode_changed = true.

During the dm_update_crtc_state disable pass we remove the stream from
the context and free it on the dm_new_crtc_state.

During the enable pass we compare the new provisional stream to the
dm_old_crtc_state->stream and determine that the stream is unchanged
and no scaling has been changed.

Following this, new_crtc_state->mode_changed is then set to false.
The connectors haven't changed and the CRTC active state hasn't changed
so drm_atomic_crtc_needs_modeset returns false, so we jump to
skip_modeset and we hit:

BUG_ON(dm_new_crtc_state->stream == NULL);

...since the old stream is gone from the context and the new stream is
also still NULL.

[How]
Ensure that we still a stream to reuse before checking if we can reuse
the old stream without a full modeset.
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -6344,7 +6344,17 @@ static int dm_update_crtc_state(struct amdgpu_display_manager *dm,
 		if (ret)
 			goto fail;
 
-		if (dc_is_stream_unchanged(new_stream, dm_old_crtc_state->stream) &&
+		/*
+		 * If we already removed the old stream from the context
+		 * (and set the new stream to NULL) then we can't reuse
+		 * the old stream even if the stream and scaling are unchanged.
+		 * We'll hit the BUG_ON and black screen.
+		 *
+		 * TODO: Refactor this function to allow this check to work
+		 * in all conditions.
+		 */
+		if (dm_new_crtc_state->stream &&
+		    dc_is_stream_unchanged(new_stream, dm_old_crtc_state->stream) &&
 		    dc_is_stream_scaling_unchanged(new_stream, dm_old_crtc_state->stream)) {
 			new_crtc_state->mode_changed = false;
 			DRM_DEBUG_DRIVER("Mode change not required, setting mode_changed to %d",