drm/i915: Ignore TMDS clock limit for DP++ when EDID override is set

4K modes testing by using dummy EDID data has never been working
properly on boxes with DP++ (dual-mode) adaptors. The reason for
this is that those modes got pruned during hdmi mode validation.
intel_hdmi_mode_valid returns CLOCK_HIGH because the pixel clock
reported by the 4k mode is higher than dual port TMDS clock limit.

However 4k injection does work properly on machines that don't have
DP++ adapters because the mode is never validated against the DP++
TMDS clock limit.

v2: Don't detect the DP++ limits when we're testing using overridden
    EDIDs. Make sure to check for the override condition after
    respecting the value of drm_dp_dual_mode_detect (Jani Nikula).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101649


Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171215102055.11729-1-abdiel.janulgue@linux.intel.com