drm/i915: properly check for MODESET for kms driver ioctls

Also ditch the cargo-culted dev_priv checks - either we have a
giant hole in our setup code or this is useless. Plainly bogus
to check for it in either case.

v2: Chris Wilson noticed that I've missed one bogus dev_priv check.

v3: The check in the overlay code is redundant (Chris)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>