LF-2744: drm: cdns: reset force_mode_set flag in atomic_check

commit 6c05ea0d158ddb1c13772cef1bd1aec0f4625664 from
https://source.codeaurora.org/external/imx/linux-imx



Reset force_mode_set flag in atomic_check function
to avoid set mode_changed flag multi times when cable plugin.

Signed-off-by: Sandor Yu <Sandor.yu@nxp.com>
Reviewed-by: Robby Cai <robby.cai@nxp.com>