radeon: fix r600/agp when vram is after AGP (v3)

If AGP is placed in the middle, the size_af is off-by-one, it results
in VRAM being placed at 0x7fffffff instead of 0x8000000.

v2: fix the vram_start setup.
v3: also fix r7xx & newer ASIC

Reported-by: russiane39 on #radeon

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