i965: Clean up misleading defines for DWORD 2 of URB_WRITE header.

i965: Clean up misleading defines for DWORD 2 of URB_WRITE header.

R02_PRIM_END and R02_PRIM_START don't actually refer to bits in DWORD
2 of R0 (as the name, and comments in the code, would seem to
indicate).  Actually they refer to bits in DWORD 2 of the header for
URB_WRITE messages.

This patch renames the defines to reflect what they actually mean.  It
also addes a define URB_WRITE_PRIM_TYPE_SHIFT, which previously was
just hardcoded in .c files.

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