drm/i915/lmem: don't treat small BAR as an error

Just pass along the probed io_size. The backend should be able to
utilize the entire range here, even if some of it is non-mappable.

It does leave open with what to do with stolen local-memory.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315181425.576828-1-matthew.auld@intel.com