Add debugfs interface to log GFXOFF statistics:

- Read amdgpu_gfxoff_count to get the total GFXOFF entry count at the
  time of query since system power-up

- Write 1 to amdgpu_gfxoff_residency to start logging, and 0 to stop.
  Read it to get average GFXOFF residency % multiplied by 100
  during the last logging interval.

Both features are designed to be keep the values persistent between
suspends.
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>