drm/i915: Use a spin lock to protect the pipe crc struct

Daniel pointed out that it was hard to get anything lockless to work
correctly, so don't even try for this non critical piece of code and
just use a spin lock.

v2: Make intel_pipe_crc->opened a bool
v3: Use assert_spin_locked() instead of a comment (Daniel Vetter)
v4: Use spin_lock_irq() in the debugfs functions (they can only be
    called from process context),
    Use spin_lock() in the pipe_crc_update() function that can only be
    called from an interrupt handler,
    Use wait_event_interruptible_lock_irq() when waiting for data in the
    cicular buffer to ensure proper locking around the condition we are
    waiting for. (Daniel Vetter)

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>