intel: restrict dma-buf-import images to external sampling only

intel: restrict dma-buf-import images to external sampling only

Memory originating outside mesa stack is meant to be for reading
only. In addition, the restrictions imposed by the image external
extension should apply. For example, users shouldn't be allowed
to generare mip-trees based on these images.

v2 (Chad): document using full extension names, fix the comment
           style itself and emit description of error

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>