drm/amd/display: Fix ASSR regression on embedded panels

commit 6be50f5 upstream.

[Why]
Regression found in some embedded panels traces back to the earliest
upstreamed ASSR patch. The changed code flow are causing problems
with some panels.

[How]
- Change ASSR enabling code while preserving original code flow
  as much as possible
- Simplify the code on guarding with internal display flag

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=213779
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1620
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -1675,9 +1675,6 @@ static enum dp_panel_mode try_enable_assr(struct dc_stream_state *stream)
 	} else
 		panel_mode = DP_PANEL_MODE_DEFAULT;
 
-#else
-	/* turn off ASSR if the implementation is not compiled in */
-	panel_mode = DP_PANEL_MODE_DEFAULT;
 #endif
 	return panel_mode;
 }