MLK-22597 drm/imx: dpu: kms: Allow primary plane on-the-fly disablement

MLK-22597 drm/imx: dpu: kms: Allow primary plane on-the-fly disablement

By correcting plane's ExtDst source in ->atomic_begin() for
cases where pixel combiner is used, commit[1] acctually fixes
the primary plane on-the-fly disablement failure issue which
commit[2] tries to address.  So, let's revert commit[2] and
allow primary plane on-the-fly disablement.

[1] commit 2f3eaadf72c3 ("MLK-22584 drm/imx: dpu: crtc: Correct plane's ExtDst source in ->atomic_begin()")
[2] commit 6477bb1492b7 ("MLK-21525 drm/imx: dpu: kms: Disallow primary plane on-the-fly disablement")

Revert "MLK-21525 drm/imx: dpu: kms: Disallow primary plane on-the-fly disablement"

This reverts commit 6477bb1492b7ac89678891447f3d794e4fdb6df6.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
(cherry picked from commit 148075a270dafa0a04f1c9b61d62e2764e469cf3)