swr: fix crash in swr_update_derived following st/mesa state changes

swr: fix crash in swr_update_derived following st/mesa state changes

Recent change to st/mesa state update logic caused major regressions to
swr validation code.

swr uses the same validation logic (swr_update_derived) for both draw
and Clear calls.  New st/mesa state update logic results in certain state
objects not being set/bound during Clear.  This was causing null ptr
exceptions.  Creation of static dummy state objects allows setting these
pointers during Clear validation, without interfering with relevant state
validation.

Once fixed, new logic also highlighted an error in dirty bit checking for
fragment shader and clip validation.

(The alternative is to have a simplified validation routine for Clear.
Which may do that at some point.)

Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>