drm/amd/display: Reset fifo after enable otg

[Why]
In fast boot sequence, when change dispclk, otg is disabled but digfe
is enabled. This may cause dig fifo error.

[How]
Reset dig fifo after enable otg.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: JinZe.Xu <JinZe.Xu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>