staging: omapdrm: Fix error paths during dmm init

Failures during the dmm probe can cause the kernel to crash.  Moved
the spinlock to a global and moved list initializations immediately
after the allocation of the dmm private structure.

Signed-off-by: Andy Gross <andy.gross@ti.com>
Reviewed-by: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>