amdgpu/dc: kill some deadcode in dc core.

Moves one function to an inline, but kills a bunch of deadcode.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>