Decrease required subgroup size

Decrease required subgroup size

dEQP-VK.spirv_assembly.instruction.spirv1p4.entrypoint.comp_workgroup_entry_point
test assumed that the subgroupSize is >= 8. The test failed if the
supported subgroupSize is less than 8.

This CL lowers the required subgroup size to 4.

Affects:
dEQP-VK.spirv_assembly.instruction.spirv1p4.entrypoint.comp_workgroup_entry_point

Components: Vulkan

VK-GL-CTS issue: 2698

Change-Id: I9931dab7a57c0b6b73530e72bca0c0d9cc90f87d
(cherry picked from commit 8f9b9137470ed5b0cd0b38edd9e9c28ccf565731)