drm/amd/display: Remove unnecessary dc_stream vtable

There's no need to have this as a vtable. The vtable was initially
used for stream_adjust_vmin_vmax but the condition checked here
(set_drr) is always true, hence we don't need to assign this
dynamically anymore.

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