st/mesa: fix crash in shader cache cased by race condition

st/mesa: fix crash in shader cache cased by race condition

If a thread doesn't load GLSL IR from cache but does load TGSI
from cache (that was created by another thread) than it will
crash due to expecting gl_program_parameter_list to have been
restored from the GLSL IR cache and not be null.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>