drm/i915: Split out load time MMIO initialization

According to the new init phases scheme we should have a definite step
in the init sequence where MMIO access is setup, so move the
corresponding code to a separate function. This also has the benefit of
making the error path cleaner both in the new function and in
i915_driver_load()/unload().

No functional change.

Suggested by Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1458128348-15730-16-git-send-email-imre.deak@intel.com