svga: init svga_screen::swc_mutex with mtx_recursive

svga: init svga_screen::swc_mutex with mtx_recursive

If the SVGA3D_BindGBSurface() call in svga_buffer_hw_storage_unmap()
fails, we'll flush and that might involve unmapping other buffers.
That leads to a recursive lock on svga_screen::swc_mutex and causes
a deadlock.  Fix this by initializing the mutex with mtx_recursive.

Note that this only happened on Linux, not Windows.  On Windows, the
mutex functions are implemented with Win32 critical sections which
support recursive locking.

Also add a comment about this.

Fixes VMware bug 1831549 (Unigine Tropics demo freeze on Linux).

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Neha Bhende<bhenden@vmware.com>