i965/blorp: Prepare for attributes other than render position

i965/blorp: Prepare for attributes other than render position

Note that the magic number of one in gen7 logic is replaced by
BRW_SF_URB_ENTRY_READ_OFFSET ( == 1 also) for clarity.

On gen6 the change from zero to one (BRW_SF_URB_ENTRY_READ_OFFSET)
has no effect for native blorp as blorp doesn't use any
additional attributes. In fact, regular pipeline setup always
uses BRW_SF_URB_ENTRY_READ_OFFSET even when there are no additional
attributes. Hence the change makes the two (blorp and regular)
consistent.

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