drm/xe: add xe_ttm_stolen_cpu_access_needs_ggtt()

xe_ttm_stolen_cpu_inaccessible() was originally meant to just cover the
case where stolen is not directly CPU accessible on some older
integrated platforms, and as such a GGTT mapping was also required for
CPU access (as per the check in xe_bo_create_pin_map_at()).

However with small-bar systems on dgfx we have one more case where
stolen is also inaccessible, however here we don't have any fallback
GGTT mode for CPU access. Fix the check in xe_bo_create_pin_map_at() to
make this distinction clear. In such a case the later vmap() will fail
anyway.

v2: fix kernel-doc warning
v3: Simplify further and remove cpu_inaccessible()

Suggested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>