drm/amd/display: Revert vblank change that causes null pointer crash

drm/amd/display: Revert vblank change that causes null pointer crash

Revert commit 1a4bcdbea431 ("drm/amd/display: Fix possible underflow for displays with large vblank")
Because it cause some regression

Fixes: 1a4bcdbea431 ("drm/amd/display: Fix possible underflow for displays with large vblank")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>