drm/amd/display: Increase Backlight Gain Step Size

[Why]
Some backlight tests fail due to backlight settling
taking too long. This happens because the step
size used to change backlight levels is too small.

[How]
1. Change the size of the backlight gain step size
2. Change how DMCU firmware gets the step size value
   so that it is passed in by driver during DMCU initn
Signed-off-by: Eryk Brol <eryk.brol@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce/dce_dmcu.c

@@ -388,6 +388,9 @@ static bool dcn10_dmcu_init(struct dmcu *dmcu)
 		/* Set initialized ramping boundary value */
 		REG_WRITE(MASTER_COMM_DATA_REG1, 0xFFFF);
 
+		/* Set backlight ramping stepsize */
+		REG_WRITE(MASTER_COMM_DATA_REG2, abm_gain_stepsize);
+
 		/* Set command to initialize microcontroller */
 		REG_UPDATE(MASTER_COMM_CMD_REG, MASTER_COMM_CMD_REG_BYTE0,
 			MCP_INIT_DMCU);

drivers/gpu/drm/amd/display/dc/dce/dce_dmcu.h

@@ -263,4 +263,6 @@ struct dmcu *dcn10_dmcu_create(
 
 void dce_dmcu_destroy(struct dmcu **dmcu);
 
+static const uint32_t abm_gain_stepsize = 0x0060;
+
 #endif /* _DCE_ABM_H_ */