drm/amdgpu: remove maximum BO size limitation v2

We can finally remove this now.

v2: remove now unused max_size variable as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>