drm/i915: Clean up i915_driver_load failure path

i915_driver_load adds a write-combining MTRR region for the GTT
aperture to improve memory speeds through the aperture. If
i915_driver_load fails after this, it would not have cleaned up the
MTRR. This shouldn't cause any problems, except for consuming an MTRR
register. Still, it's best to clean up completely in the failure path,
which is easily done by calling mtrr_del if the mtrr was successfully
allocated.

i915_driver_load calls i915_gem_load which register
i915_gem_inactive_shrink. If i915_driver_load fails after calling
i915_gem_load, the shrinker will be left registered. When called, it
will access freed memory and crash. The fix is to unregister the shrinker in the
failure path using code duplicated from i915_driver_unload.

i915_driver_load also has some incorrect gotos in the error cleanup
paths:

 * After failing to initialize the GTT (which cannot happen, btw,
   intel_gtt_get returns a fixed (non-NULL) value), it tries to
   free the uninitialized WC IO mapping. Fixed this by changing the
   target from out_iomapfree to out_rmmap

Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Lin Ming <ming.m.lin@intel.com>