r600g: suballocate memory for the STRMOUT_BUFFER_FILLED_SIZE register

r600g: suballocate memory for the STRMOUT_BUFFER_FILLED_SIZE register

Instead of having a 4-byte buffer for each streamout target, we suballocate
each dword from a 4K buffer.

This further reduces the overall number of relocations.

Tested-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>