drm/i915/tgl: Fix TC-cold block/unblock sequence

The command register is the PCODE MBOX low register not the high one as
described by the spec. This left the system with the TC-cold power state
being blocked all the time. Fix things by using the correct register.

Also to make sure we retry a request for at least 600usec, when the
PCODE MBOX command itself succeeded, but the TC-cold block command
failed, sleep for 1msec unconditionally after any fail.

The change was tested with JTAG register read of the HW/FW's actual
TC-cold state, which reported the expected states after this change.

Tested-by: Nivedita Swaminathan <nivedita.swaminathan@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200805150056.24248-1-imre.deak@intel.com