drm/amd/display: Amend coasting vtotal for replay low hz

[WHY]
The original coasting vtotal is 2 bytes, and it need to
be amended to 4 bytes because low hz case.

[HOW]
Amend coasting vtotal from 2 bytes to 4 bytes.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: ChunTao Tso <chuntao.tso@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>