drm/v3d: Remove some dead members of struct v3d_bo.

vmas was from the previous model of page table management (one per
fd), and vaddr was left over from vc4.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308161716.2466-4-eric@anholt.net
Acked-by: Rob Herring <robh@kernel.org>