drm/i915: use might_lock_nested in get_pages annotation

So strictly speaking the existing annotation is also ok, because we
have a chain of

obj->mm.lock#I915_MM_GET_PAGES -> fs_reclaim -> obj->mm.lock

(the shrinker cannot get at an object while we're in get_pages, hence
this is safe). But it's confusing, so try to take the right subclass
of the lock.

This does a bit reduce our lockdep based checking, but then it's also
less fragile, in case we ever change the nesting around.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191104173720.2696-3-daniel.vetter@ffwll.ch