radv: do not use radeon_set_context_reg_seq() for only one register
radeon_set_context_reg() is a shortcut for that. Signed-off-by:Samuel Pitoiset <samuel.pitoiset@gmail.com> Reviewed-by:
Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl> Part-of: <mesa/mesa!12099>