drm/i915: Extract i915_gem_fence.c

No code changes, just moving all the fence related code into a
separate file (and avoiding a bunch of forward declarations while at
it).

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

diff --cc drivers/gpu/drm/i915/Makefile
Simple merge

diff --cc drivers/gpu/drm/i915/i915_drv.h
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem.c
Simple merge

diff --cc drivers/gpu/drm/i915/i915_gem_fence.c
Simple merge