Refactor X11 and wayland platforms to run alongside each other

Refactor X11 and wayland platforms to run alongside each other

This adds a capability for running Vulkan WSI ane EGL tests on both
X11 and wayland compositors from one binary built in 'default' target.

The change replaces X11 & wayland tcu::Platform-s with one common
'linux' platform, that is parametrized by DEQP_SUPPORT_
(X11|GLX|XCB|WAYLAND) at compile time.

This new platform supports creating WSI displays and EGL contexts
for each of compositors enabled at build-time. To choose EGL compositor
use --deqp-egl-display-type=x11 or Wayland.

No functional change is made to 'wayland' target: it is still possible
to build Wayland-only binary using that target.

VK-GL-CTS issue: 83
Components: Framework
Affects: dEQP-VK.wsi.* and all tests using EGL.

Change-Id: I1d8725d1329e1071ac545f765c374d2d7d5a29df