glsl: Switch to disable-by-default for the GLSL shader cache

glsl: Switch to disable-by-default for the GLSL shader cache

The shader cache is expected to be developed incrementally over a
fairly long series of commits. For that period of instability, we
require users to opt into the shader cache by setting:

	MESA_GLSL_CACHE_ENABLE=1

In the future, when the shader cache is complete, we can revert this
commit so that the cache will be on by default.

The user can always disable the cache with
MESA_GLSL_CACHE_DISABLE=1. That functionality is not affected by this
commit, (nor will it be affected by the future revert).

Reviewed-by: Eric Anholt <eric@anholt.net>