winsys/amdgpu: avoid double simple_mtx_unlock()

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.

This occurs in amdgpu_winsys_create() after
cb446dc0
winsys/amdgpu: Add amdgpu_screen_winsys

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Cc: 19.2 19.3 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 3fe3bde4)