drm/amd/display: Fix Warnings

- Remove mixed declarations and unused variables

Signed-off-by: Amy Zhang <Amy.Zhang@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>