i965: De-duplicate key_debug() function.

i965: De-duplicate key_debug() function.

This appeared in brw_vs.c and brw_wm.c, should have appeared in
brw_gs.c, and was soon going to have to be in brw_tcs.c and brw_tes.c as
well.

So, instead, move it to a central location (which has to know about both
struct brw_context and perf_debug()).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>