egl: implement new functions from EGL_EXT_image_flush_external

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-By: Tapani Pälli <tapani.palli@intel.com>

src/egl/main/egldisplay.h

@@ -105,6 +105,7 @@ struct _egl_extensions
    EGLBoolean EXT_create_context_robustness;
    EGLBoolean EXT_image_dma_buf_import;
    EGLBoolean EXT_image_dma_buf_import_modifiers;
+   EGLBoolean EXT_image_flush_external;
    EGLBoolean EXT_pixel_format_float;
    EGLBoolean EXT_surface_CTA861_3_metadata;
    EGLBoolean EXT_surface_SMPTE2086_metadata;