drm/i915: Align initial plane backing objects correctly

Some bios really like to joke and start the planes at an offset ...
hooray!

Align start and end to fix this.

v2: Fixup calculation of size, spotted by Chris Wilson.

v3: Fix serious fumble I've just spotted.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86883
Cc: stable@vger.kernel.org
Cc: Johannes W <jargon@molb.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reported-and-tested-by: Johannes W <jargon@molb.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
[Jani: split WARN_ONs, rebase on v4.0-rc1]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>