drm/i915: Clean conflicting modesetting registers upon init

If we leave the registers in a conflicting state then when we attempt
to teardown the active mode, we will not disable the pipes and planes
in the correct order -- leaving a plane reading from a disabled pipe and
possibly leading to undefined behaviour.

Reported-and-tested-by: Andy Whitcroft <apw@canonical.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=32078


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org