drm/radeon: properly filter DP1.2 4k modes on non-DP1.2 hw

The check was already in place in the dp mode_valid check, but
radeon_dp_get_dp_link_clock() never returned the high clock
mode_valid was checking for because that function clipped the
clock based on the hw capabilities.  Add an explicit check
in the mode_valid function.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=87172

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc:stable@vge.kernel.org

drivers/gpu/drm/radeon/atombios_dp.c

@@ -492,6 +492,10 @@ int radeon_dp_mode_valid_helper(struct drm_connector *connector,
 	struct radeon_connector_atom_dig *dig_connector;
 	int dp_clock;
 
+	if ((mode->clock > 340000) &&
+	    (!radeon_connector_is_dp12_capable(connector)))
+		return MODE_CLOCK_HIGH;
+
 	if (!radeon_connector->con_priv)
 		return MODE_CLOCK_HIGH;
 	dig_connector = radeon_connector->con_priv;