drm/i915: Disable high-bpc on pre-1.4 EDID screens

Prevents black screens when using 30bpp framebuffers on my
HDMI screens here. The DP input on the same screen though reports a
1.4 EDID with the correct 8bpc limit set.

v2: Actually check for the right thing!

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>