drm/i915/dp: do not proceed with autotests if we don't ACK them

There is no point in setting intel_dp->compliance.test_type, and
proceeding with the autotests, if we're about to NAK the request. Some
drive-by cleanups while at it.

v2: look at the ACK bit, as the result may also contain
    TEST_EDID_CHECKSUM_WRITE

Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Tested-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484931846-25390-1-git-send-email-jani.nikula@intel.com