drm/i915: Add a delay in Displayport AUX transactions for compliance testing

The Displayport Link Layer Compliance Testing Specification 1.2 rev 1.1
specifies that repeated AUX transactions after a failure (no response /
invalid response) must have a minimum delay of 400us before the resend can
occur. Tests 4.2.1.1 and 4.2.1.2 are two tests that require this specifically.

Also, the check for DP_AUX_CH_CTL_TIME_OUT_ERROR has been moved out into a
separate case. This case just continues with the next iteration of the loop
as the HW has already waited the required amount of time.

V2:
- Changed udelay() to usleep_range()
V3:
- Removed extraneous check for timeout
- Updated comment to reflect this change
V4:
- Reformatted a comment
V5:
- Added separate check for HW timeout on AUX transactions. A message
  is logged upon detection of this case.
V6:
- Add continue statement to HW timeout detect case
- Remove the log message indicating a timeout has been
  detected (review feedback)
V7:
- Updated the commit message to remove verbage about the HW timeout
  case that is no longer valid.

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>