drm/amdgpu/display: restore AUX_DPHY_TX_CONTROL for DCN2.x

Commit 0982149 added fetching of the AUX_DPHY register
values from the vbios, but it also changed the default values
in the case when there are no values in the vbios.  This causes
problems with displays with high refresh rates.  To fix this,
switch back to the original default value for AUX_DPHY_TX_CONTROL.

Fixes: 0982149 ("drm/amd/display: Read VBIOS Golden Settings Tbl")
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1426
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Igor Kravchenko <Igor.Kravchenko@amd.com>
Cc: Aric Cyr <Aric.Cyr@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/display/dc/dcn20/dcn20_link_encoder.c

@@ -341,8 +341,7 @@ void enc2_hw_init(struct link_encoder *enc)
 	} else {
 		AUX_REG_WRITE(AUX_DPHY_RX_CONTROL0, 0x103d1110);
 
-		AUX_REG_WRITE(AUX_DPHY_TX_CONTROL, 0x21c4d);
-
+		AUX_REG_WRITE(AUX_DPHY_TX_CONTROL, 0x21c7a);
 	}
 
 	//AUX_DPHY_TX_REF_CONTROL'AUX_TX_REF_DIV HW default is 0x32;