drm/amdgpu: re-apply "move internal vram_mgr function into the C file""

This re-applys commit 708d19d9.

The original problem this was reverted for was found and the correct fix
will be merged to drm-misc-next-fixes.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220714132315.587217-2-christian.koenig@amd.com