drm/i915: Add missing include file <linux/math64.h>

Fix build error:
./drivers/gpu/drm/i915/selftests/i915_random.h: In function i915_prandom_u32_max_state:
./drivers/gpu/drm/i915/selftests/i915_random.h:48:23: error:
 implicit declaration of function mul_u32_u32; did you mean mul_u64_u32_div? [-Werror=implicit-function-declaration]
  return upper_32_bits(mul_u32_u32(prandom_u32_state(state), ep_ro));

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 7ce5b685

 ("drm/i915/selftests: Use mul_u32_u32() for 32b x 32b -> 64b result")
Signed-off-by: YueHaibing <yuehaibing@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/20200107135014.36472-1-yuehaibing@huawei.com