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>