egl/dri2: implement platform_surfaceless

egl/dri2: implement platform_surfaceless

The surfaceless platform is for off-screen rendering only. Render node support
is required.

Only consider the render nodes. Do not use normal nodes as they require
auth hooks.

v3: change platform_null to platform_surfaceless
v4: make libdrm required for surfaceless
v5: remove modified include guards with defined(HAVE_SURFACELESS_PLATFORM)
v6: use O_CLOEXEC for drm fd

Signed-off-by: Haixia Shi <hshi@chromium.org>
Signed-off-by: Zach Reizner <zachr@google.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chad Versace <chad.versace@intel.com>