egl: reset blob cache set/get functions on terminate

Fixes errors seen with eglSetBlobCacheFuncsANDROID on Android when
running dEQP that terminates and reinitializes a display.

Fixes: 6f5b5709 "egl: add support for EGL_ANDROID_blob_cache"
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 3e03a3fc)