mesa: use _mesa_clear_texture_image() in clear_texture_fields()

mesa: use _mesa_clear_texture_image() in clear_texture_fields()

This avoids a failed assert(img->_BaseFormat != -1) in
init_teximage_fields_ms() because the internalFormat argument is GL_NONE.
This was hit when using glTexStorage() to do a proxy texture test.

Fixes a failure with the updated Piglit tex3d-maxsize test.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>