drm/i915: Change locking for struct_mutex, v3.

struct_mutex is being locked for every plane in intel_prepare_plane_fb
and intel_cleanup_plane_fb.

Require the caller to hold the mutex, and only acquire the mutex for
each helper call. This way the lock only needs to be acquired
twice in ->atomic_commit(). Once for pinning new framebuffers at the
start, the second time for unpinning old framebuffer.

Changes since v1:
- Use mutex_lock_interruptible instead of i915 variant,
  to prevent a deadlock when called from the reset code.
Changes since v2:
- Clarify struct_mutex is locked by the caller.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com> #v1
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>