drm/amd/display: set dig fifo read start level to 7 before dig fifo reset

[Why]
DIG_FIFO_ERROR = 1 caused mst daisy chain 2nd monitor black.

[How]
We need to set dig fifo read start level = 7 before dig fifo reset during dig
fifo enable according to hardware designer's suggestion. If it is zero, it will
cause underflow or overflow and DIG_FIFO_ERROR = 1.
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Wang Fudong <Fudong.Wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn32/dcn32_dio_stream_encoder.c

@@ -310,6 +310,11 @@ static void enc32_stream_encoder_dp_unblank(
 	// TODO: Confirm if we need to wait for DIG_SYMCLK_FE_ON
 	REG_WAIT(DIG_FE_CNTL, DIG_SYMCLK_FE_ON, 1, 10, 5000);
 
+	/* read start level = 0 will bring underflow / overflow and DIG_FIFO_ERROR = 1
+	 * so set it to 1/2 full = 7 before reset as suggested by hardware team.
+	 */
+	REG_UPDATE(DIG_FIFO_CTRL0, DIG_FIFO_READ_START_LEVEL, 0x7);
+
 	REG_UPDATE(DIG_FIFO_CTRL0, DIG_FIFO_RESET, 1);
 
 	REG_WAIT(DIG_FIFO_CTRL0, DIG_FIFO_RESET_DONE, 1, 10, 5000);