drm/i915: Add automated testing support for Displayport compliance testing

Add the skeleton framework for supporting automation for Displayport compliance
testing. This patch adds the necessary framework for the source device to
appropriately respond to test automation requests from a sink device.

V2:
- Addressed previous mailing list feedback
- Fixed compilation issue (struct members declared in a later patch)
- Updated debug messages to be more accurate
- Added status checks for the DPCD read/write calls
- Removed excess comments and debug messages
- Fixed debug message compilation warnings
- Fixed compilation issue with missing variables
- Updated link training autotest to ACK

V3:
- Fixed the checks on the DPCD return code to be <= 0
  rather than != 0
- Removed extraneous assignment of a NAK return code in the
  DPCD read failure case
- Changed the return in the DPCD read failure case to a goto
  to the exit point where the status code is written to the sink
- Removed FAUX test case since it's deprecated now
- Removed the compliance flag assignment in handle_test_request

V4:
- Moved declaration of type_type here
- Removed declaration of test_data (moved to a later patch)
- Added reset to 0 for compliance test variables

V5:
- Moved test_active variable declaration and initialization out of
  this patch and into the patch where it's used
- Changed variable name compliance_testing_active to
  compliance_test_active to unify the naming convention
- Added initialization for compliance_test_type variable

Signed-off-by: Todd Previte <tprevite@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>