drm/amdgpu: drop dead header

Not used anymore.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Noticed-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>