drm/i915: Add debugfs test control files for Displayport compliance testing

drm/i915: Add debugfs test control files for Displayport compliance testing

This patch adds 3 debugfs files for handling Displayport compliance testing
and supercedes the previous patches that implemented debugfs support for
compliance testing. Those patches were:

- [PATCH 04/17] drm/i915: Add debugfs functions for Displayport
                          compliance testing
- [PATCH 08/17] drm/i915: Add new debugfs file for Displayport
                          compliance test control
- [PATCH 09/17] drm/i915: Add debugfs write and test param parsing
                          functions for DP test control

This new patch simplifies the debugfs implementation by places a single
test control value into an individual file. Each file is readable by
the usersapce application and the test_active file is writable to
indicate to the kernel when userspace has completed its portion of the
test sequence.

Replacing the previous files simplifies operation and speeds response
time for the user app, as it is required to poll on the test_active file
in order to determine when it needs to begin its operations.

V2:
- Updated the test active variable name to match the change in
  the initial patch of the series
V3:
- Added a fix in the test_active_write function to prevent a NULL pointer
  dereference if the encoder on the connector is invalid

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>