drm/nvc0/fb: take lock in nvc0_ram_put()

Kernel panic caused by list corruption in ltcg seems to indicate a
concurrency issue.

Take mutex of pfb like nv50_ram_put() to eliminate concurrency.

V2: Separate critical section into separate function, avoid taking the
    lock twice on NVC0

Signed-off-by: Roy Spliet <r.spliet@student.tudelft.nl>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>