drm/i915: Remove too early plane enable on pre-PCH hardware

Enabling the plane before we have assigned valid address means that it
will access random PTE (often with conflicting memory types) and cause
GPU lockups. However, enabling the plane too early appears to workaround
a number of bugs in our modesetting code.

Cc: Franz Melchior <melchior.franz@gmail.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=39947
References: https://bugs.freedesktop.org/show_bug.cgi?id=41091
References: https://bugs.freedesktop.org/show_bug.cgi?id=49041


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>