drm/amd/display: Fix compiler redefinition warnings for certain configs

[why & how]
Modified definitions of 1 function and 2 structs to remove warnings on
certain specific compiler configurations due to redefinition.

Reviewed-by: Martin Leung <martin.leung@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Mounika Adhuri <moadhuri@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>