drm/i915: sprinkle static

Apparently we've missed a few more than what Fengguang's 0-day tester
recently reported in i915_irq.c ... Makes sparse happy again (ignore
some spurious stuff about ksyms of exported functions).

Cc: kbuild test robot <fengguang.wu@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>