Fix uniform subgroup size test

Fix uniform subgroup size test

The uniform subgroup size test was requiring the subgroup size to be
uniform across several vkCmdDispatch calls, which is not required by the
spec. Furthermore, both dispatch calls were using different local sizes
in the shader.

In addition, these changes add a missing barrier from compute to host.

Components: Vulkan
VK-GL-CTS issue: 3505

Affected tests:
dEQP-VK.subgroups.multiple_dispatches.uniform_subgroup_size

Change-Id: Icea2479da1793f0b24f6fb8a4dd60d477ef8a775