[Linux] Enabling Hardware Acceleration with Libva in linux-x86.

This commit is not upstreamable.

Currently, VAAPI only enabled for ChromeOS and Windows on
upstream. Although the ChromeOS uses same libva API as
common linux world, upstream refuesed to enable it for linux
common with maintainence concern. Refer to
"https://codereview.chromium.org/16430003/" for more details.

Upstream confirm VAVDA will continue to be restricted to
CrOS/X86 for dev & testing only and not for chromium road map.
So we maintain it for Crosswalk and that will benifit
linux with VAAPI installed and includes Tizen.
[Tizen] Enabling Hardware Acceleration with Libva and EGL in
     VDA for Tizen Mobile within chromium v32+.

Why we need to maintain it in our side:
===========================================
1) Upstream confirm VAVDA will continue to be restricted to
   CrOS/X86 for dev & testing only and not for chromium road map.
2) CrOS/X86 no plan to expend EGL backend which finalize in
   June 2012 and be addressed to the CrOS graphics team.

So, the upstream no plan to lerage VAVDA with EGL graphic
   backend for any X86.

3) The tizen-mobile's driver only support EGL as texture
   backend. The video hw acceleration of xwalk have to
   rely on EGL not GLX to bind decoded pixmap.
===========================================
That's why we enable specific EGL for VAVDA in tizen port.