i965: don't use disk cache with SPIR-V shaders

i965: don't use disk cache with SPIR-V shaders

Right now we don't support disk cache for SPIR-V shaders (from
ARB_gl_spirv), so let's avoid writing the program data to or reading
it from the disk if any in-use shaders use SPIR-V.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>