Validate instancing cases properly

Validate instancing cases properly

The vertex attrib 64bit limits test case executes
both regular and instanced draw calls and has a
validation function that takes a boolean parameter
that informs it about whether isntancing has been
used so it can verify that all instances executed
properly. However, this function is always called
with false as parameter, indicating no instancing,
even for instanced scenarios, which means that the
validation function is only validating the first
instance even for instanced cases.

Components: OpenGL
VK-GL-CTS issue: 916

Affects:
KHR-GL45.vertex_attrib_64bit.limits_test
KHR-GL46.vertex_attrib_64bit.limits_test

Change-Id: Ibaf2c8fc0b6599a17f7bdbc7c4914b42d38d5e2a