drm/i915: Allocate the PCI resource for the MCHBAR

We were failing when trying to allocate the resource for MMIO of the
MCHBAR because we forgot to specify what type of resource we wanted.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>