drm/i915: Split GGTT initialisation between probing and setup

In order to handle conflicting drivers (i.e. vgacon) having a different
setup of hardware, we have to remove those other drivers before we try
to setup our own mappings. This requires us to split GGTT initialisation
between probing for the hardware location (part of the PCI BAR) and
later establishing the kernel resources for it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-2-git-send-email-chris@chris-wilson.co.uk