drm/etnaviv: use new iterator in etnaviv_gem_describe

Instead of hand rolling the logic.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025080532.177424-2-christian.koenig@amd.com