i965: Initialize disk shader cache if MESA_GLSL_CACHE_DISABLE is false

i965: Initialize disk shader cache if MESA_GLSL_CACHE_DISABLE is false

(Apologies for the double negative.)

For now, the shader cache is disabled by default on i965 to allow us
to verify its stability.

In other words, to enable the shader cache on i965, set
MESA_GLSL_CACHE_DISABLE to false or 0. If the variable is unset, then
the shader cache will be disabled.

We use the build-id of i965_dri.so for the timestamp, and the pci
device id for the device name.

v2:
 * Simplify code by forcing link to include build id sha. (Matt)

v3:
 * Don't use a for loop with snprintf for bin to hex. (Matt)
 * Assume fixed length render and timestamp string to further simplify
   code.

Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>