drm/i915: Fix multiple definition of 'i915_vma_capture_finish'

If 'CONFIG_DRM_I915_CAPTURE_ERROR' not configured, there are some
errors like:

drivers/gpu/drm/i915/i915_irq.o:
	In function `i915_vma_capture_finish':
./drivers/gpu/drm/i915/i915_gpu_error.h:312:
	multiple definition of `i915_vma_capture_finish'
drivers/gpu/drm/i915/i915_drv.o:
	./drivers/gpu/drm/i915/i915_gpu_error.h:312: first defined here

So, add 'static inline' on the defineation of the 'i915_vma_capture_finish'

Fixes: d713e3ab

("drm/i915: Correct typo in i915_vma_compress_finish stub")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200113104009.13274-1-zhangxiaoxu5@huawei.com