drm/amdgpu/gmc: move vram type fetching into sw_init

early_init gets called before atom asic init so on non-posted
cards, the vram type is not initialized.

Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org