drm/radeon: fix problem with changing active VRAM size. (v2)

drm/radeon: fix problem with changing active VRAM size. (v2)

So we used to use lpfn directly to restrict VRAM when we couldn't
access the unmappable area, however this was removed in
6bc283cd4061b1c0215691a9a9e7029dc9b2f799 as it also restricted
the gtt placements. However it was only later noticed that this
broke on some hw.

This removes the active_vram_size, and just explicitly sets it
when it changes, TTM/drm_mm will always use the real_vram_size,
and the active vram size will change the TTM size used for lpfn
setting.

We should re-work the fpfn/lpfn to per-placement at some point
I suspect, but that is too late for this kernel.

Hopefully this addresses:
https://bugs.freedesktop.org/show_bug.cgi?id=35254

v2: fix reported useful VRAM size to userspace to be correct.

Signed-off-by: Dave Airlie <airlied@redhat.com>