drm/i915: Apply lockdep annotations to i915_gem_context.c

Markup the functions that require the caller to hold struct_mutex with
lockdep_assert_held(). In the hopefully not-too-distant future we will
split the struct_mutex up, and in doing so we need to be sure that we
know what it protects - here the lockdep annotations are invaluable.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-2-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-2-git-send-email-chris@chris-wilson.co.uk