drm/amdgpu: fix vm size and block size for VMPT (v5)

Set reasonable defaults per family.

v2: set both of them in gmc
v3: move vm size and block size in vm manager
v4: squash in warning fix from Alex Xie
v5: squash in min() warning fix

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