i965/sf: Use BRW_SF_URB_ENTRY_READ_OFFSET rather than hardcoded values.

i965/sf: Use BRW_SF_URB_ENTRY_READ_OFFSET rather than hardcoded values.

We always program the SF unit to start reading the vertex URB entry at
offset 1.  In upcoming patches, we'll be adding FS code that relies on
this.  So consistently use the constant BRW_SF_URB_ENTRY_READ_OFFSET
rather than hardcoding a 1.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>