drm/amd/powerplay: add interface to init fb allocations (v2)

This patch adds interface to init fb allocations for smu.
It's to allocate vram bos to store smc table contents.

v2: add detailed info to describe this function

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>