drm/i915: Insert i915_preliminary_hw_support variable.

On the worst scenario, users with new hardwares and old kernel from
enabling times can get black screens.  So, from now on, this
perliminary_hw_support module parameter shall be used by all upcoming
platforms that are still under enabling. The second option would be to
merge the pci ids after basic modeset works, but that makes testing
and development while bringing up hw a rather tedious afair.

Although it is uncomfortable for developers use this extra variable it
brings more stability for end users.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
[danvet: dropped the i915_ param prefix, i915.i915_ is just tedious.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>