drm/i915: Disable display when fused off

FUSE_STRAP has a bit to inform us that the display has been fused off.
Use it to setup the definitive number of pipes at run-time.

v2: actually tweak num_pipes, not num_planes
v3: also tests SFUSE_STRAP bit 7
v4: rebase on top of drm-nightly
    use DRM_INFO() for the message telling display is fused off
    try to read the FUSE_LOCK bit to determine if PCH display is disabled
v5: Don't read SFUSE_STRAP (register on the PCH) if num_pipes is already 0
    from the initial device info struct (to prevent hangs) (Daniel Vetter)

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com> (for v3)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (for v3)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>