gpu_display: Fix variable type
gpu_display: Fix variable type

... to fix Android build.

BUG=b:173523402
TEST=build

Change-Id: Ifee00141eb804f31361aa1d7bdec2ff9e391f52d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2964312
Auto-Submit: Jason Macnak <76caa58940b8c6785e996f522e85b625ac7c7baa@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Gurchetan Singh <99434bba058e25b6203b8ae055a4f697dad0ef64@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm