drm/amd/display: Explicitly cast v_total to signed in a subtraction

drm/amd/display: Explicitly cast v_total to signed in a subtraction

[WHY & HOW]
v_total is an uint32_t and subtracting an unsigned to a signed will
result in an unsigned which is always >= 0. As a result, the ternary
conditions are always true and thus has no effect.

This is fixed by casting v_total to signed explicitly. This also
avoids v_total subtraction to overflow.

This fixes 1 NO_EFFECT, 2 DEADCODE and 2 INTEGER_OVERFLOW issues reported
by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@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>