drm/i915/display: Record the plane update times for debugging

Since we try and estimate how long we require to update the registers to
perform a plane update, it is of vital importance that we measure the
distribution of plane updates to better guide our estimate. If we
underestimate how long it takes to perform the plane update, we may
slip into the next scanout frame causing a tear. If we overestimate, we
may unnecessarily delay the update to the next frame, causing visible
jitter.

Replace the warning that we exceed some arbitrary threshold for the
vblank update with a histogram for debugfs.

v2: Add a per-crtc debugfs entry so that the information is easier to
extract when testing individual CRTC, and so that it can be reset before
a test.

v3: Flip the graph on its side; creates space to label the time axis.

Updates: 4684
       |
   1us |
       |
   4us |********
       |**********
  16us |***********
       |*****
  66us |
       |
 262us |
       |
   1ms |
       |
   4ms |
       |
  17ms |
       |
Min update: 5918ns
Max update: 54781ns
Average update: 16628ns
Overruns > 250us: 0

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1982


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20201202212814.26320-1-chris@chris-wilson.co.uk