drm/v3d: replace open-coded implementation of drm_gem_object_lookup

As v3d_submit_tfu_ioctl() performs the same steps as
drm_gem_object_lookup(), replace the open-code implementation in v3d
with its DRM core equivalent.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221227200010.191351-1-mcanal@igalia.com