drm/i915: Fix locking in i915_dma.c

Some i915_dma.c functions didn't include locking directives or used
them in a different order than Linux.
cyapa - Add sysctl for minimum pressure

* Add a sysctl for minimum pressure to detect a finger.  Fingers below this
  value will be ignored.  Default to 16.
ig4 - Fix minor documentation bug

* Fix minor doc bug in register definitions.