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>