drm/i915: Drop checks for initialization

KMS always intializes, this was only a valid check when userspace
was still in control of the kernel driver.

v2: Comment that we outright reject all dri1/ums params.

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