drm/amdgpu: Introduce functions for vram physical addr calculation

Add one function to calculate BO's GPU physical address.
And another function to calculate BO's CPU physical address.

v2: Use functions vs macros (Christian)
    Use more proper function names (Christian)

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>