amdgpu: remove invalid check in amdgpu_bo_alloc

The heap is checked by the kernel and not libdrm, to make it even worse
it prevented allocating resources other than VRAM and GTT.

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