drm/amd/display: Make functional resource functions non-static

[Why & How]
To align coding style for how we use this across DCN. The resource
creation ones can remain static, however.

Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>