backend/drm: fix atomic commits when the GAMMA_LUT prop is missing
backend/drm: fix atomic commits when the GAMMA_LUT prop is missing

We already have the logic to fallback to the legacy interface above. We
just need to avoid calling atomic_add with a zero prop ID.

Closes: https://github.com/swaywm/wlroots/issues/2187

swaywm/wlroots,ascent12/wlroots,ascent12/wlroots,SirCmpwn/wlroots,swaywm/wlroots