drm/i915/gsc: Fix the Driver-FLR completion

The Driver-FLR flow may inadvertently exit early before the full
completion of the re-init of the internal HW state if we only poll
GU_DEBUG Bit31 (polling for it to toggle from 0 -> 1). Instead
we need a two-step completion wait-for-completion flow that also
involves GU_CNTL. See the patch and new code comments for detail.
This is new direction from HW architecture folks.

   v2: - Add error message for the teardown timeout (Anshuman)
       - Don't duplicate code in comments (Jani)
   v3: - Add get/put runtime-pm for this function. Though
         not functionally required during unload, its so the uncore
	 doesn't complain.
   v4: - Remove the get/put runtime-pm - that was for a prior
         version of this patch (not needed for drm-managed callback).
       - Remove the fixes tag since this is only for MTL and MTL
         still needs force probe (Daniele).
       - Bit 31 of GU_CNTL should be DRIVERFLR instead of
         DRIVERFLR_STATUS (Daniele).

Signed-off-by: Alan Previn <[email protected]>
Tested-by: Vinay Belgaumkar <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Signed-off-by: John Harrison <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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