egl/main: Enable Linux platform extensions

egl/main: Enable Linux platform extensions

Enable EGL_EXT_platform_base and the Linux platform extensions layered
atop it: EGL_EXT_platform_x11, EGL_EXT_platform_wayland,
and EGL_MESA_platform_gbm.

Tested with Piglit's EGL_EXT_platform_base tests under an X11 session.
To enable running the Wayland and GBM tests, windowed Weston was running
and the kernel had render nodes enabled.

I regression tested my EGL_EXT_platform_base patch set with Piglit on
Ivybridge under X11/EGL, standalone Weston, and GBM with rendernodes. No
regressions found.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>