drm/i915/ilk: Move register read under spinlock

Code does read-modify-write but the read was outside the lock.

It is fine since the caller holds struct mutex, but if we
correct this we open up the opportunity for decreasing the
mutex duration time since the call to ironlake_enable_drps
does not need it any longer since it is covered by the
mchdev_lock lock.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1455186452-13691-2-git-send-email-tvrtko.ursulin@linux.intel.com