drm: use new iterator in drm_gem_plane_helper_prepare_fb v3

Makes the handling a bit more complex, but avoids the use of
dma_resv_get_excl_unlocked().

v2: improve coding and documentation
v3: adjust the TODO comment as suggested by Daniel

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-25-christian.koenig@amd.com