drm/i915: Convert intel_overlay.c to use native drm_i915_private pointers

drm/i915: Convert intel_overlay.c to use native drm_i915_private pointers

Another day, another long overdue conversion. Not much to update inside
intel_overlay.c, but still

   text	   data	    bss	    dec	    hex	filename
6309547	3578778	 696320	10584645	 a18245	vmlinux
6309291	3578778	 696320	10584389	 a18145	vmlinux

a couple of hundred bytes of pointer misdirection.

Whilst here, rename the ioctl entry points to include the _ioctl suffix
so that the user entry points are clear (following the idiom).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1463053403-25086-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>