drm/amdgpu: fix vbios fetching for SR-IOV

SR-IOV fetches the vbios from VRAM in some cases.
Re-enable the VRAM path for dGPUs and rename the function
to make it clear that it is not IGP specific.

Fixes: 042658d1 ("drm/amdgpu: clean up vbios fetching code")
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Tested-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>