mesa: Mark Identity as const

mesa: Mark Identity as const

I was going to send this as review for dce1e1a8, but I missed that
window.  This saves 64 bytes of unshared data and prelaces it with 96
bytes shared text.  My guess is that some of the calls to memcpy get
optimized to something else.

   text	   data	    bss	    dec	    hex	filename
7847613	 220208	  27432	8095253	 7b8615	i965_dri.so before
7847709	 220144	  27432	8095285	 7b8635	i965_dri.so after

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: Brian Paul <brianp@vmware.com>