drm/i915: Make i945gm_vblank_work_func static

drm/i915: Make i945gm_vblank_work_func static

drivers/gpu/drm/i915/i915_irq.c:3382:6: warning: symbol 'i945gm_vblank_work_func' was not declared. Should it be static?
  CC [M]  drivers/gpu/drm/i915/i915_irq.o
drivers/gpu/drm/i915/i915_irq.c:3382:6: error: no previous prototype for ‘i945gm_vblank_work_func’ [-Werror=missing-prototypes]
 void i945gm_vblank_work_func(struct work_struct *work)

Jani wrote the idential patch, so for posterity:

The static keyword was apparently accidentally removed in commit
71cef97d39c6 ("drm/i915: Switch to per-crtc vblank vfuncs"), leading to
sparse warning:

drivers/gpu/drm/i915/i915_irq.c:3382:6: warning: symbol
'i945gm_vblank_work_func' was not declared. Should it be static?

Make the function static again.

Meanwhile, the 0-day kbuilder also spotted the mistake.

Fixes: 71cef97d39c6 ("drm/i915: Switch to per-crtc vblank vfuncs")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626224212.10141-1-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20190627091914.30795-1-jani.nikula@intel.com