drm/xe/stolen: don't map stolen on small-bar

The driver should still be functional with small-bar, just that the vram
size is clamped to the BAR size (until we add proper support for tiered
vram). For stolen vram we shouldn't iomap anything if the BAR size
doesn't also contain the stolen portion, since on discrete the stolen
portion is always at the end of normal vram. Stolen should still be
functional, just that allocating CPU visible io memory will always
return an error.

v2 (Lucas)
  - Mention in the commit message that stolen vram is always as the end
    of normal vram, which is why stolen in not mappable on small-bar
    systems.
  - Just make xe_ttm_stolen_inaccessible() return true for such cases.
    Also rename to xe_ttm_stolen_cpu_inaccessible to better describe
    that we are talking about direct CPU access. Plus add some
    kernel-doc.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/209


Reported-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>