drm/amd/display: Correct unit conversion for vstartup

[Why]
vstartup is calculated to be a large number. It works because
it is within vertical blank, but it reduces region of blank that
can be used for power gating.

[How]
Calculation needs to convert micro seconds to number of
vertical lines.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Reza Amini <reza.amini@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>