mesa: add a function to do the image data copy stuff for save_CompressedTex(Sub)Image

mesa: add a function to do the image data copy stuff for save_CompressedTex(Sub)Image

Introuduce a simple function called copy_data to do the image data copy
stuff for all the save_CompressedTex*Image function. The function check
the NULL data case to avoid some potential segfault. This also would
make the code a bit simpler and less redundance.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>