drm/amdgpu: query vram_width from vram_info table

Driver will get channel_number and channel_width from
vram_info table, then calculate vram_width by multiply
channel_number by channel_width

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>