drm/i915: Rearrange GGTT probing to avoid needing a vfunc

Since we have a static if-else-chain for device probing of the global
GTT, we do not need to use a function pointer, let alone store it when
we never use it again. So use the if-else-chain to call down into the
device specific probe.

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/1470293567-10811-5-git-send-email-chris@chris-wilson.co.uk