Fix using YV12 as a format instead of type.

Bug 25563277

Change-Id: I1721f060e313c135e713915ddd2dbdbca5239380
Reviewed-on: https://swiftshader-review.googlesource.com/4221
Reviewed-by: Keun Soo Yim <yim@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>