drm/amd/display: Guard DML_FAIL_DSC_VALIDATION_FAILURE

[Why]
Usage of this enum is DSC-only.

[How]
Guard it with CONFIG_DRM_AMD_DC_DSC_SUPPORT.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>