drm/radeon: don't init gpuvm if accel is disabled (v3)

If acceleration is disabled, it does not make sense
to init gpuvm since nothing will use it.  Moreover,
if radeon_vm_init() gets called it uses accel to try
and clear the pde tables, etc. which results in a bug.

v2: handle vm_fini as well
v3: handle bo_open/close as well

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=88786



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org