broadcom/vc4: Fix use-after-free when deleting a program.

broadcom/vc4: Fix use-after-free when deleting a program.

By leaving the compiled shader in the context's stage state, the next
compile of a new FS would look in the old compiled FS for figuring out
whether to set various dirty flags for the VS compile.  Clear out the
pointer when deleting the program, and make sure that we always mark the
state as dirty if the previous program had been lost.  Fixes valgrind
warnings on glsl-max-varyings.

Fixes: 2350569a78c6 ("vc4: Avoid VS shader recompiles by keeping a set of FS inputs seen so far.")