drm/i915/gt: Fix reservation address in ggtt_reserve_guc_top

There is an assertion in ggtt_reserve_guc_top that the global GTT
is of size at least GUC_GGTT_TOP, which is not the case on a 32-bit
platform; see commit 562d55d9
("drm/i915/bdw: Only use 2g GGTT for 32b platforms"). If GEM_BUG_ON
is enabled, this triggers a BUG(); if GEM_BUG_ON is disabled, the
subsequent reservation fails and the driver fails to initialise
the device:

i915 0000:00:02.0: [drm:i915_init_ggtt [i915]] Failed to reserve top of GGTT for GuC
i915 0000:00:02.0: Device initialization failed (-28)
i915 0000:00:02.0: Please file a bug on drm/i915; see https://gitlab.freedesktop.org/drm/intel/-/wikis/How-to-file-i915-bugs for details.
i915: probe of 0000:00:02.0 failed with error -28

Make the reservation at the top of the available space, whatever
that is, instead of assuming that the top will be GUC_GGTT_TOP.

Fixes: 91180076 ("drm/i915/uc: Reserve upper range of GGTT")
Link: https://gitlab.freedesktop.org/drm/intel/-/issues/9080


Signed-off-by: Javier Pello <devel@otheo.eu>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Fernando Pacheco <fernando.pacheco@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: stable@vger.kernel.org # v5.3+
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230902171039.2229126186d697dbcf62d6d8@otheo.eu