drm/amd/display: remove redundant vertical_line_start variable

Return value from expression directly instead of
taking this in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jinpeng Cui <cui.jinpeng2@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>