drm/amd/display: Clear underflow status for debug purposes

We want to keep underflow sticky bit on for the longevity tests outside
of test environment. For debug purposes it is, however, useful to clear
underflow status after the test that caused it so that the following
tests are not affected. This change fullfils both requirements by clearing
the underflow only from within Windows or Diags test environment.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>