drm/amdkcl: Fix 64 bit wraparound resulting in illegal drm mode

[Why]
For m = drm_display_mode{.clock = 533250, .htotal = 4000, .vtotal = 2222},
common_rates[i] = 60000, the result of target_vtotal is 2221. This cause
wraparound of variable target_vtotal_diff.

[How]
Skip the loop if target_vtotal less than m->vtotal

Signed-off-by: Leslie Shi <Yuliang.Shi@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>