drm/i915: Simplify DP vs. eDP detection

Reduce the eDP detection to just checking if it's port A, or if
the VBT tells us that the port is eDP for the other ports.

Suggested-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>