UtilsVk::clearFramebuffer was not setting the sample count.

Bug: angleproject:3204
Change-Id: Iab7d385ec58b12f21d888c5dfc9e94b7846ac40c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1616858Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>