If we need to make room for some mappable object, then we should
only victimize objects that have one or pages that occupy the visible
portion of LMEM. Let's also create a new priority hint for objects that
are placed in mappable memory, where we know that CPU access was
requested, that way we hopefully victimize these last.

v2(Thomas): s/TTM_PL_PRIV/I915_PL_LMEM0/

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20220228123607.580432-1-matthew.auld@intel.com