drm/i915: add SFUSE_STRAP registers for digital port detection

DDIA is detected via the DDI_BUF_CTL registers bit 0, but for DDIB, DDIC
and DDID we need to consult SFUSE_STRAP values.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>