drm/amd/display: Round calculated vtotal

drm/amd/display: Round calculated vtotal

[WHY]
The calculated vtotal may has 1 line deviation. To get precisely
vtotal number, round the vtotal result.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Signed-off-by: Robin Chen <robin.chen@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>