backend-drm: always create gem_handle_refcnt hash table

I came across this when trying a fairly old version of libmali on rk3399 (https://github.com/JeffyCN/mirrors/blob/libmali/lib/aarch64-linux-gnu/libmali-midgard-t86x-r18p0-wayland-gbm.so to be exact). I didn't figure out exactly what combination of DRM/other devices exposed it on that before abandoning the whole attempt, so the commit message is lacking on why this has gone unnoticed in two years. Still, it looks like an obvious omission and did get me a step further in that attempt.