drm/amdgpu: dGPU mode set VRAM range lpfn as exclusive

TTM place lpfn is exclusive used as end (start + size) in drm and buddy
allocator, adev->gmc memory partition range lpfn is inclusive (start +
size - 1), should plus 1 to set TTM place lpfn.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>