drm/amd/display: fix link_validation build failure

When CONFIG_DRM_AMD_DC_DCN is disabled, the is_frl member
is not defined:

drivers/gpu/drm/amd/amdgpu/../display/dc/link/link_validation.c: In function 'dp_active_dongle_validate_timing':
drivers/gpu/drm/amd/amdgpu/../display/dc/link/link_validation.c:126:66: error: 'const struct dc_dsc_config' has no member named 'is_frl'
  126 |                         if (timing->flags.DSC && !timing->dsc_cfg.is_frl)
      |                                                                  ^

Use the same #ifdef as the other references to this.

Fixes: 54618888 ("drm/amd/display: break down dc_link.c")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/link_validation.c

@@ -123,9 +123,11 @@ static bool dp_active_dongle_validate_timing(
 		if (dongle_caps->dp_hdmi_frl_max_link_bw_in_kbps > 0) { // DP to HDMI FRL converter
 			struct dc_crtc_timing outputTiming = *timing;
 
+#if defined(CONFIG_DRM_AMD_DC_DCN)
 			if (timing->flags.DSC && !timing->dsc_cfg.is_frl)
 				/* DP input has DSC, HDMI FRL output doesn't have DSC, remove DSC from output timing */
 				outputTiming.flags.DSC = 0;
+#endif
 			if (dc_bandwidth_in_kbps_from_timing(&outputTiming) > dongle_caps->dp_hdmi_frl_max_link_bw_in_kbps)
 				return false;
 		} else { // DP to HDMI TMDS converter