drm/i915: Move MCHBAR setup earlier during init

Move the MCHBAR setup right after the MMIO setup, since the two things
are logically related and the MCHBAR setup code doesn't depend on any
other device specific resource. We'll also need MCHBAR to be ready
earlier in an upcoming patch, so this is also a preparation for that.

Factor out the init/clean-up code to separate functions to make things
clearer in the i915_driver_load()/unload() functions.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453209992-25995-7-git-send-email-imre.deak@intel.com