drm/amd/display: Clean up flip pending timeout handling

[Why]
Adjust timeout handling code for easier runtime manipulation
during debug. Change has no functional effect by default.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>