Gfx-3d: fix GetProcAddress in egl extention

This patch can fix issue when Proc address return NULL.
Glbench10, egl-ext and conformance test need this patch to run pass.

Change-Id: I6b9fe5f04901510440cbcbfeec947621ef72ec23
Author: Liuhui Lu <liuhui.lu@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 52622