drm/amd/display: Revert check for flip pending before locking pipes

[Why]
Causes underflow regression

[How]
This reverts commit 99d1437a

Signed-off-by: Isabel Zhang <isabel.zhang@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>