i965: Make brw_clear_cache NULL out stale program pointers.

The L3 partitioning code tries to look at all programs - both render
programs (VS/TCS/TES/GS/FS) and compute (CS).

After calling brw_clear_cache, all prog_data pointers are invalid and
point to freed data.  The intention was that flagging the dirty bits for
all programs would cause the next draw call to re-run the atoms for each
program stage, uploading new programs and installing new, valid pointers.

However, this doesn't quite work in our new multi-pipeline world.  When
drawing or dispatching a compute workload, we only consider the programs
for the appropriate pipeline: drawing sets up VS/TCS/TES/GS/FS, but not
CS, and vice versa.  This leaves pointers dangling a bit longer than
intended.

The L3 configuration code tries to inspect the prog_data for all shader
stages, so that we avoid having to reconfigure it when swapping back and
forth between render and compute workloads.  So we can't have dangling
pointers.

The fix is simple: have brw_clear_cache NULL out stale prog_data
pointers, making it safe to inspect.  The next L3 configuration pass
will see either the render shaders or compute shader as missing for
one go around, but will pick them up when both pipelines have run.

In other words, we'll simply reconfigure L3 twice, which is safe,
if a tiny bit wasteful - but then again, we just threw every compiled
shader we had on the floor and started recompiling the from scratch,
which is massively more wasteful, so it's not much of a concern.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93790
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Francisco Jerez <[email protected]>
Reviewed-by: Jordan Justen <[email protected]>

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