drm/i915/gsc: Do a driver-FLR on unload if GSC was loaded

If the GSC was loaded, the only way to stop it during the driver unload
flow is to do a driver-FLR.
The driver-initiated FLR is not the same as PCI config space FLR in
that it doesn't reset the SGUnit and doesn't modify the PCI config
space. Thus, it doesn't require a re-enumeration of the PCI BARs.
However, the driver-FLR does cause a memory wipe of graphics memory
on all discrete GPU platforms or a wipe limited to stolen memory
on the integrated GPU platforms.

We perform the FLR as the last action before releasing the MMIO bar, so
that we don't have to care about the consequences of the reset on the
unload flow.

v2: rename FLR function, add comment to explain FLR impact (Rodrigo),
    better explain why GSC needs FLR (Alan)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208200521.2928378-5-daniele.ceraolospurio@intel.com