drm/radeon/kms: avoid corner case issue with unmappable vram V2

drm/radeon/kms: avoid corner case issue with unmappable vram V2

We should not allocate any object into unmappable vram if we
have no means to access them which on all GPU means having the
CP running and on newer GPU having the blit utility working.

This patch limit the vram allocation to visible vram until
we have acceleration up and running.

Note that it's more than unlikely that we run into any issue
related to that as when acceleration is not woring userspace
should allocate any object in vram beside front buffer which
should fit in visible vram.

V2 use real_vram_size as mc_vram_size could be bigger than
   the actual amount of vram

[airlied: fixup r700_cp_stop case]

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