drm/v3d: Fix prime imports of buffers from other drivers.

drm/v3d: Fix prime imports of buffers from other drivers.

v3d_bo_get_pages() checks this to decide to map the imported buffer
instead of the backing shmem file.  The caller was about to set this
value anyway, and there's no error path in between.  Ideally we
wouldn't even allocate the shmem file for our imports, but that's a
more invasive fix.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 57692c94dcbe ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Link: https://patchwork.freedesktop.org/patch/msgid/20181128230927.10951-3-eric@anholt.net
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Dave Emett <david.emett@broadcom.com>