drm/v3d: Clean up the reservation object setup.

The extra to_v3d_bo() calls came from copying this from the vc4
driver, which stored the cma gem object in the structs.

v2: Fix an unused var warning

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181108161654.19888-4-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com> (v1)