drm/amd/display: Restore guard against default backlight value < 1 nit

Mark reports that brightness is not restored after Xorg dpms screen blank.

This behavior was introduced by commit d9e8658 ("drm/amd/display:
Simplify brightness initialization") which dropped the cached backlight
value in display code, but also removed code for when the default value
read back was less than 1 nit.

Restore this code so that the backlight brightness is restored to the
correct default value in this circumstance.

Reported-by: Mark Herbert <[email protected]>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3031
Cc: [email protected]
Cc: Camille Cho <[email protected]>
Cc: Krunoslav Kovac <[email protected]>
Cc: Hamza Mahfooz <[email protected]>
Fixes: d9e8658 ("drm/amd/display: Simplify brightness initialization")
Acked-by: Alex Deucher <[email protected]>
Signed-off-by: Mario Limonciello <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

drivers/gpu/drm/amd/display/dc/link/protocols/link_edp_panel_control.c

@@ -287,8 +287,8 @@ bool set_default_brightness_aux(struct dc_link *link)
 	if (link && link->dpcd_sink_ext_caps.bits.oled == 1) {
 		if (!read_default_bl_aux(link, &default_backlight))
 			default_backlight = 150000;
-		// if > 5000, it might be wrong readback
-		if (default_backlight > 5000000)
+		// if < 1 nits or > 5000, it might be wrong readback
+		if (default_backlight < 1000 || default_backlight > 5000000)
 			default_backlight = 150000;
 
 		return edp_set_backlight_level_nits(link, true,