r600g, radeonsi: fix primitives-generated query with disabled streamout

r600g, radeonsi: fix primitives-generated query with disabled streamout

Buffers are disabled by VGT_STRMOUT_BUFFER_CONFIG, but the query only works
if VGT_STRMOUT_CONFIG.STREAMOUT_0_EN is enabled.

This moves VGT_STRMOUT_CONFIG to its own state. The register is set to 1
if either streamout or the primitives-generated query is enabled.

However, the primitives-emitted query is also incremented, so it's disabled
by setting VGT_STRMOUT_BUFFER_SIZE to 0 when there is no buffer bound.

This fixes piglit:
  ARB_transform_feedback2/counting with pause
  EXT_transform_feedback/primgen-query transform-feedback-disabled

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>