drm/i915: Split out load time HW initialization

According to the new init phases scheme we should have a definite step
in the init sequence where we setup things requiring accessing the
device, so move the corresponding code to separate function. The steps
in this init phase should avoid exposing the driver via some interface,
which is done in the last registration init phase. This changae also
has the benefit of making the error path cleaner both in the new
function and 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/1458128348-15730-17-git-send-email-imre.deak@intel.com