i965/fs: Properly check for PAD in fragment shaders with > 16 varyings.

i965/fs: Properly check for PAD in fragment shaders with > 16 varyings.

Commit 268008f98c3810b9f276df985dc93efc0c49f33e changed unused VUE map
slots to be initialized with BRW_VARYING_SLOT_PAD, not COUNT.  I missed
updating this.  It also means that commit message was wrong, as some
code *did* rely slots being initialized to COUNT.

This may fix a bug with SSO programs with > 16 FS input varyings.
I think we probably just emitted extra pointless code, but probably
didn't break anything.  We might also just have no tests for that.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>