egl/dri2: Don't check return result of mtx_unlock().

Coverity (CID 1358496) warns that the cleanup code doesn't unlock the
mutex (which is arguably kind of stupid, since the only case that can
happen is when mtx_unlock() failed!). But, mtx_unlock() isn't going to
fail -- the mutex was locked by this thread just a few lines above it.
1 file changed