drm/i915: Add a spinlock to protect the active_list

This is a baby-step in the direction of having finer-grained
locking than the struct_mutex. Specifically, this will enable
new debugging code to read the active list for printing out
GPU state when the GPU is wedged, (while the struct_mutex is
held, of course).

Signed-off-by: Carl Worth <cworth@cworth.org>
[anholt: indentation fix]
Signed-off-by: Eric Anholt <eric@anholt.net>