drm/i915: Move Displayport test request and sink IRQ logic to intel_dp_detect()

drm/i915: Move Displayport test request and sink IRQ logic to intel_dp_detect()

Due to changes in the driver and to support Displayport compliance testing,
the test request and sink IRQ logic has been relocated from
intel_dp_check_link_status to intel_dp_detect. This is because the bulk of the
compliance tests that set the TEST_REQUEST bit in the DEVICE_IRQ field of the
DPCD issue a long pulse / hot plug event to signify the start of the test.
Currently, for a long pulse, intel_dp_check_link_status is not called for a
long HPD pulse, so if test requests come in, they cannot be detected by the
driver.

Once located in the intel_dp_detect, in the regular hot plug event path,
proper detection of Displayport compliance test requests occurs which then
invokes the test handler to support them. Additionally, this places compliance
testing in the normal operational paths, eliminating as much special case code
as possible.

The only change in intel_dp_check_link_status with this patch is that when
the IRQ is the result of a test request from the sink, the test handler is not
invoked during the short pulse path. Short pulse test requests are for a
particular variety of tests (mainly link training) that will be implemented
in the future. Once those tests are available, the test request handler will
be called from here as well.

V2:
- Rewored the commit message to be more clear about the content and intent
  of this patch
- Restore IRQ detection logic to intel_dp_check_link_status(). Continue to
  detect and clear sink IRQs in the short pulse case. Ignore test requests
  in the short pulses for now since they are for future test implementations.

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>