Fix MCHBAR cleanup on the driver init error path

MCHBAR is cleaned up in i915_mmio_cleanup(), so the separate call in
i915_driver_load() is incorrect.

CC: David Weinehall <david.weinehall@intel.com>
Fixes: ad5c3d3f ("drm/i915: Move MCHBAR setup earlier during init")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-2-git-send-email-imre.deak@intel.com