drm/i915: Check for IVB instead of gen7 when we think about IVB CPU eDP

Almost all of the GEN7 checks in the DP code are actually looking for
IVB. HSW doesn't even take these codepaths, and VLV is excluded on
account of not having port A. So let's change the checks to IS_IVB to
make the code less confusing.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517170309.28630-3-ville.syrjala@linux.intel.com