r600g: Add pkt_flag member to struct r600_command_buffer

r600g: Add pkt_flag member to struct r600_command_buffer

Some packets require the shader type bit (bit 1) to be set when
used for compute shaders.  The pkt_flag will be initialized to
RADEON_CP_PACKET3_COMPUTE_MODE for any struct r600_command_buffer used
for dispatching compute shaders and it will be or'd against the result of
the PKT3 macro when adding a new packet to a struct r600_command buffer.

Reviewed-by: Marek Olšák <maraeo@gmail.com>