drm/amd/display: Change disable backlight ramp change threshold from 0 to maximum value.

Instead of user set brightness with range of percentage,
HLK test set brightness level with range of normal, this will result in
HLK test case set brightness from 0 to 255, DC set brightness with ramp is 0,
and disabled ramp change which will fail the HLK test.
Fix:
In case of unblank stream and turn on edp, change brightness level in
stream to 0xFFFFFFFF(actural maximum level is 0xFF), use that value as
a flag to recogonize this the case of resume from S3.
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1982,7 +1982,7 @@ bool dc_link_set_backlight_level(const struct dc_link *link, uint32_t level,
 		return false;
 
 	if (stream) {
-		if (stream->bl_pwm_level == 0)
+		if (stream->bl_pwm_level == EDP_BACKLIGHT_RAMP_DISABLE_LEVEL)
 			frame_ramp = 0;
 
 		((struct dc_stream_state *)stream)->bl_pwm_level = level;

drivers/gpu/drm/amd/display/dc/dce110/dce110_hw_sequencer.c

@@ -1036,7 +1036,7 @@ void dce110_unblank_stream(struct pipe_ctx *pipe_ctx,
 
 	if (link->local_sink && link->local_sink->sink_signal == SIGNAL_TYPE_EDP) {
 		link->dc->hwss.edp_backlight_control(link, true);
-		stream->bl_pwm_level = 0;
+		stream->bl_pwm_level = EDP_BACKLIGHT_RAMP_DISABLE_LEVEL;
 	}
 }
 void dce110_blank_stream(struct pipe_ctx *pipe_ctx)

drivers/gpu/drm/amd/display/dc/inc/hw_sequencer.h

@@ -32,6 +32,8 @@
 #include "inc/hw/link_encoder.h"
 #include "core_status.h"
 
+#define EDP_BACKLIGHT_RAMP_DISABLE_LEVEL 0xFFFFFFFF
+
 enum pipe_gating_control {
 	PIPE_GATING_CONTROL_DISABLE = 0,
 	PIPE_GATING_CONTROL_ENABLE,