drm/panfrost: Align GEM objects GPU VA to 2MB

In order to increase the chances of using 2MB pages, we need to align the
GPU VA mapping to 2MB. Only do this if the object size is 2MB or more.

Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190610165806.24854-1-robh@kernel.org