drm/amdgpu: Fix hardcoded base offset of vram pages

In gmc_v9_0_vram_gtt_location(),the vram_base_offset is hardcoded
to 0 in dGPU. Fix it by reading mmMC_VM_FB_OFFSET or return
zfb_phys_addr if ZFB is enabled.

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>