drm/gem: Allow sparsely populated page arrays in drm_gem_put_pages

Panfrost has a need for pages allocated on demand via GPU page faults.
When releasing the pages, the only thing preventing using
drm_gem_put_pages() is needing to skip over unpopulated pages, so allow
for skipping over NULL struct page pointers.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808222200.13176-2-robh@kernel.org

drivers/gpu/drm/drm_gem.c

@@ -633,6 +633,9 @@ void drm_gem_put_pages(struct drm_gem_object *obj, struct page **pages,
 
 	pagevec_init(&pvec);
 	for (i = 0; i < npages; i++) {
+		if (!pages[i])
+			continue;
+
 		if (dirty)
 			set_page_dirty(pages[i]);