i965: Don't add base_binding_table_index if it's zero

i965: Don't add base_binding_table_index if it's zero

When calculating the binding table index for non-constant sampler
array indexing it needs to add the base binding table index which is a
constant within the generated code. Often this base is zero so we can
avoid a redundant instruction in that case.

It looks like nothing in shader-db is doing non-constant sampler array
indexing so this patch doesn't make any difference but it might be
worth having anyway.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Acked-by: Ben Widawsky <ben@bwidawsk.net>