drm/amd/display: Use amdgpu mode funcs statically

No need to assign them dynamically. This is much more readable.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>