backend/drm: Use legacy gamma size for legacy backend
backend/drm: Use legacy gamma size for legacy backend

We would always return the GAMMA_LUT_SIZE property if available, and
only fall back to legacy gamma size otherwise. This leads to issues if
both are available in differs in size while we use the legacy backend.

Ensure that we only return the legacy size if we're using the legacy
backend.

Closes: https://github.com/swaywm/wlroots/issues/2429

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots