Make create_buffer test use platform limits

Make create_buffer test use platform limits

The create_buffer tests use a maximum buffer size of half the heap size
as reported by vkGetPhysicalDeviceMemoryProperties. On systems where
device memory comes out of system memory, due to the memory in use in
the system the selected buffer size may be too large.

The heap size is checked against the platform limits, the min is taken
and that is used as the basis for calculating the buffer size to test

Affects:

dEQP-VK.api.buffer.createBuffer_*

Components: Vulkan

VK-GL-CTS issue: 362

Change-Id: I7a644e8e48b961dad0d19675e3d54add4b11d99c