i965/blorp: Expose the shader cache through function pointers

i965/blorp: Expose the shader cache through function pointers

This sanitizes blorp's access to the i965 driver's shader cache by patching
it through the blorp_context.  When we start using blorp in Vulkan, we will
simply have to implement such a caching interface in the Vulkan driver.

Note: In my first attempt at this, I simplified it down to a single
upload_shader entrypoint and implemented the caching inside of blorp.  This
doesn't work, however, because the i965 driver will, on occation, dump its
entire cache and start over.  When this happens, blorp needs to be able to
recompile its shaders and re-upload them.  It's easiest to just expose the
caching interface.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>