i965: Make a helper for finding an existing shader variant.

i965: Make a helper for finding an existing shader variant.

We had five copies of the same "walk the cache and look for an
existing shader variant for this program" code.  Now we have one
helper function that returns the key.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>