drm/i915: Fixup stub definitions for intel_opregion_suspend|resume

drm/i915: Fixup stub definitions for intel_opregion_suspend|resume

248 "multiple definition of ...".  E.g.:

    LD [M]  drivers/gpu/drm/i915/i915.o
  ld: drivers/gpu/drm/i915/i915_irq.o: in function `intel_opregion_resume':
  i915_irq.c:(.text+0x58f0): multiple definition of `intel_opregion_resume';
  drivers/gpu/drm/i915/i915_drv.o:i915_drv.c:(.text+0x2d40): first defined here

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: a950adc6c343 ("drm/i915: Stop calling intel_opregion unregister/register in suspend/resume")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build tested
Link: https://patchwork.freedesktop.org/patch/msgid/20181129210543.31555-1-chris@chris-wilson.co.uk