drm/amdgpu/gmc9: set vram_width properly for SR-IOV

For SR-IOV, vram_width can't be read from ATOM as
RAVEN, and DF related registers is not readable, so hardcord
is the only way to set the correct vram_width.

Reviewed-by: Yintian Tao <yttao@amd.com>
Signed-off-by: Trigger Huang <Trigger.Huang@amd.com>
Signed-off-by: Yintian Tao <yttao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>