Simplify isDeviceFunctionalitySupported
Simplify isDeviceFunctionalitySupported

Add comments explaining the two parts of the check and simplify the
checking of extensions that the device reports are supported -- the
feature bits are compulsory when the extension is supported.

Components: Vulkan
VK-GL-CTS issue: 3149
Affects: dEQP-VK.*

Change-Id: Iec81cdf588146d2adb07871b5950024d3cdcf189

KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS