drm/i915: Add support for DP link training compliance

This patch adds support to handle automated DP compliance
link training test requests. This patch has been tested with
Unigraf DPR-120 DP Compliance device for testing Link
Training Compliance.
After we get a short pulse Compliance test request, test
request values are read and hotplug uevent is sent in order
to trigger another modeset during which the pipe is configured
and link is retrained and enabled for link parameters requested
by the test.

v5:
* Only modify the compliance structure after all validation
is done (Jani Nikula)
* Remove the variable test_result (Jani Nikula)
v4:
* Return TEST_NAK for read failures and invalid
values (Jani Nikula)
* Conver the test link BW to link rate before storing (Jani Nikula)
v3:
* Validate the test link rate and lane count as soon as
the request comes (Jani Nikula)
v2:
* Validate the test lane count before using it in
intel_dp_compute_config (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/1485274594-17361-1-git-send-email-manasi.d.navare@intel.com