drm/i915: Fix warning in i915_gem_chipset_flush

drivers/gpu/drm/i915/i915_drv.h:1545:2: warning: '______f' is static but
declared in inline function 'i915_gem_chipset_flush' which is not static

Reported-by: kbuild test robot <fengguang.wu@intel.com>
dri-devel-Reference: <50a4d41c.586VhmwghPuKZbkB%fengguang.wu@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>