drm/i915: don't allow interlaced pipeconf on gen2

gen2 doesn't support it, so be a bit more paranoid and add a check to
ensure that we never ever set an unsupported interlaced bit.

Ensure that userspace can't set an interlaced mode by resetting
interlace_allowed for the crt on gen2. dvo and lvds are the only other
encoders that gen2 supports and these already disallow interlaced
modes.

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>