egl/dri2: avoid undefined unlocks

egl/dri2: avoid undefined unlocks

unlocks were incorrectly added to paths using dri2_egl_display() as
well as those using dri2_egl_display_lock()

pthread_mutex_unlock() when unlocked is documented by posix as
being undefined behaviour.  On OpenBSD pthread_mutex_unlock() will call
abort(3) if this happens.

Fixes: f1efe037dfd ("egl/dri2: Add display lock")
Reviewed-by: Rob Clark <robclark@freedesktop.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20712>
(cherry picked from commit 0594b3c143d48c5f37f705abb03f18fbbabbc6b0)