drm/i915: Fixes to support DP Compliance EDID tests

This patch addresses a few issues from the original patch for
DP Compliance EDID test support submitted by
Todd Previte<todd.previte@gmail.com>

Video Mode requested in the EDID test handler for the EDID Read
test (CTS 4.2.2.3) should be set to PREFERRED as per the CTS spec.

v2:
* Added read debugfs data from test_data.edid if its EDID test (Jani NIkula)
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484968170-12467-3-git-send-email-manasi.d.navare@intel.com