drm/amdgpu: update ib_start/size_alignment same as windows used

PAGE_SIZE for start_alignment is far much than hw requirement,
And now, update to expereince value from window side.

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>