drm/i915: Promote i915->mm.obj_lock to be irqsafe

The intent is to be able to update the mm.lists from inside an irqsoff
section (e.g. from a softirq rcu workqueue), ergo we need to make the
i915->mm.obj_lock irqsafe.

v2: can_discard_pages() ensures we are shrinkable
v3: Beware shadowing of 'flags'

Fixes: 3b4fa964 ("drm/i915: Track the purgeable objects on a separate eviction list")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110869


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610145430.17717-1-chris@chris-wilson.co.uk