drm/i915/selftests: handle allocation failures

drm/i915/selftests: handle allocation failures

If we have to contend with non-mappable LMEM, then we need to ensure the
object fits within the mappable portion, like in the selftests, where we
later try to CPU access the pages. However if it can't then we need to
gracefully handle this, without throwing an error.

Also it looks like TTM will return -ENOMEM, in ttm_bo_mem_space() after
exhausting all possible placements.

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>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220228123607.580432-3-matthew.auld@intel.com