teximage: split out helper from EGLImageTargetTexture2DOES

The major differences between EXT_EGL_image_storage and
EGLImageTargetTexture2DOES are:

(1) The texture target is made immutable
(2) EXT_EGL_image_storage supports non-2D targets.

We can reuse EGLImageTargetTexture2D and FreeTextureImageBuffer
for (1) pretty easily.  For (2), let's just not support the
complicated targets.  Let's reuse aspects of the
EGLImageTargetTexture2DOES implementation.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>