drm/i915: Refuse modes with hdisplay==4096 on pre-HSW DP

drm/i915: Refuse modes with hdisplay==4096 on pre-HSW DP

The DP port/pipe goes wonky if we try to use timings with
hdisplay==4096 on pre-HSW platforms. The link fails to train
and the pipe may not signal vblank interrupts. On HDMI such at
mode works just fine (tested on ELK/SNB/CHV). So let's refuse
such modes on DP on older platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718144340.1114-1-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>