drm/amd/display: Fix enum decl warnings

drm/amd/display: Fix enum decl warnings

[Why]
warnings were triggered by enum forward declarations that are not
valid in C++ without an explicit underlying type.

[How]
- Replace problematic enum forward declarations with C++-safe forms where
  applicable.
- Use plain integer types for interface-only declarations that do not
  require strong enum typing.
- Update dependent winterface signatures and related type usage
  consistently.
- Add required include and type-visibility fixes to avoid follow-on parse
  and type-resolution issues.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Gaghik Khachatrian <gaghik.khachatrian@amd.com>
Signed-off-by: James Lin <pinglei.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>