i965: Remove caching of surface state objects.

i965: Remove caching of surface state objects.

It turns out that computing a 56 byte key to look up a 20-byte object
out of a hash table was some sort of a bad idea.  Whoops.

before:
[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]       gl            firefox-talos-gfx   37.799   38.203   0.39%    6/6
after:
[  0]       gl            firefox-talos-gfx   34.761   34.784   0.17%    5/6