drm/v3d: replace obj lookup steps with drm_gem_objects_lookup

As v3d_lookup_bos() performs the same steps as drm_gem_objects_lookup(),
replace the explicit code in v3d to simply use the DRM function.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221205135538.3545051-3-mwen@igalia.com