drm/xe/gsc: Do not attempt to load the GSC multiple times

The GSC HW is only reset by driver FLR or D3cold entry. We don't support
the former at runtime, while the latter is only supported on DGFX, for
which we don't support GSC. Therefore, if GSC failed to load previously
there is no need to try again because the HW is stuck in the error state.

An assert has been added so that if we ever add DGFX support we'll know
we need to handle the D3 case.

v2: use "< 0" instead of "!= 0" in the FW state error check (Julia).

Fixes: dd0e89e5

 ("drm/xe/gsc: GSC FW load")
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: <stable@vger.kernel.org> # v6.8+
Reviewed-by: Julia Filipchuk <julia.filipchuk@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240828215158.2743994-2-daniele.ceraolospurio@intel.com
(cherry picked from commit 2160f6f6e3cf6893a83357c3b82ff8589bdc0f08)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>