drm/i915/flipq: Implement Wa_18034343758

Implement the driver side of Wa_18034343758, which is supposed to
prevent the DSB and DMC from accessing registers in parallel, and
thus potentially corrupting the registers due to a hardware issue
(which should be fixed in PTL-B0).

The w/a sequence goes as follows:
DMC starts the DSB
 |                 \
DMC halts itself    | DSB waits a while for DMC to have time to halt
 .                  | DSB executes normally
 .		    | DSB unhalts the DMC at the very end
 .             	   /
DMC resumes execution

v2: PTL-B0+ firmware no longer has the w/a since the hw got fixed
v3: Do the w/a on all PTL for now since we only have the A0 firmware
    binaries which issues the halt instructions unconditionally
v4: PTL DMC binaries do in fact have the A0 vs. B0 split, so skip
    the w/a on PTL-B0+

Reviewed-by: Uma Shankar <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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