drm/amd/display: Add SPL namespace

[Why]
In order to avoid component conflicts, spl namespace is needed.

[How]
Adding SPL namespace to the public API os that each user of SPL can have
their own namespace.

Signed-off-by: Navid Assadian <Navid.Assadian@amd.com>
Reviewed-by: Samson Tam <Samson.Tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>