The value of some magic regsiters differ across chipsets. fd6_context
manages the differences by initializing them at runtime. Let's do the
same.

Add to tu_physical_device a subset of those found in fd6_context:

    RB_UNKNOWN_8E04_blit
    RB_CCU_CNTL_gmem
    PC_UNKNOWN_9805
    SP_UNKNOWN_A0F8

Part-of: <mesa/mesa!3743>