drm/vc4: replace obj lookup steps with drm_gem_objects_lookup

As vc4_cl_lookup_bos() performs the same steps as drm_gem_objects_lookup(),
replace the open-coded implementation in vc4 to simply use the DRM function.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230202111943.111757-3-mcanal@igalia.com