drm/i915/gsc: Fix race between HW init and GSC FW load

The GSC FW load is a slow process (up to 250 ms), so we defer it to a
dedicated worker to avoid stalling the init flow for that long. However,
we currently start this worker before the HW init is complete, so there
is a chance that the GSC loading code submits to the HW before the
engine initialization has completed. We can easily fix this by starting
the thread later in the gt_resume flow.
From this later spot, the GSC code can still race with the default
submission code; we functionally don't care who wins the race (the GSC
load doesn't need any state), but since the whole point of the separate
worker is to make the main thread faster, we prefer the default
submission code to run first. Therefore, make an exception for driver
probe and only and start the gsc load from uc_init_late.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230223172120.3304293-3-daniele.ceraolospurio@intel.com