i965: Tidy preprocessor macros for SO_NUM_PRIMS_WRITTEN registers.

i965: Tidy preprocessor macros for SO_NUM_PRIMS_WRITTEN registers.

Gen7+ supports four transform feedback streams.  Using a function-like
macro makes it easy to access them by stream number or loop over them.
"GEN7_" prefixes are more common than "_IVB" suffixes, so we use that.

Gen6 only supports a single stream, so the single #define should be
fine.  However, SO_NUM_PRIMS_WRITTEN was confusingly generic, as it
doesn't exist on Gen7+.  Add a "GEN6_" prefix for clarity.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>