radv: optimize radv_stage_flush() for pre fragment shader stages

radv: optimize radv_stage_flush() for pre fragment shader stages

We don't need to emit PS_PARTIAL_FLUSH for the pre fragment shader
stages (ie. geometry/tessellation). Emitting VS_PARTIAL_FLUSH
is enough for these stages. Note that PS_PARTIAL_FLUSH also
synchronizes all vertex stages.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>