drm/msm/dp: skip PUSH_IDLE when the link was never enabled

In the Linux kernel, the following vulnerability has been resolved:

drm/msm/dp: skip PUSH_IDLE when the link was never enabled

msm_dp_display_atomic_enable() returns early when link training fails,
leaving ->power_on false and the main link down.
msm_dp_display_atomic_disable() nevertheless writes DP_STATE_CTRL_PUSH_IDLE
and waits for an idle-pattern completion that cannot arrive, so every failed
enable is followed by "PUSH_IDLE pattern timedout".

Every other step of the teardown is already gated on that flag:
msm_dp_display_disable(), called from .atomic_post_disable(), returns early
on !power_on. The PUSH_IDLE write is the only one that is not, so the
controller's runtime-PM reference is then dropped without the link having
been taken down.

On glymur (Snapdragon X2 Elite) the consequence is not a warning. The SoC
does not survive it: TrustZone force-stops the SOCCP and ADSP remote
processors and the machine resets silently about 50 ms later, with no oops
and no panic. On an ASUS Zenbook A16 (UX3607OA), whose eDP panel does not
currently train, this reproduces without any compositor or GPU involvement:

# eDP enable has already failed with "Failed link training (rc=-104)"
echo 1 > /sys/class/graphics/fb0/blank

[535.645455] === marker ===
[535.694833] qcom_q6v5_pas d00000.remoteproc: fatal error received: \
sys_m_smsm.c:512:TZ force stop
[535.694875] remoteproc remoteproc0: crash detected in soccp: type fatal error
[535.728857] qcom_q6v5_pas 6800000.remoteproc: fatal error received: \
sys_m_smsm.c:783:err fatal notification received from TZ
<SoC reset>

Gate the PUSH_IDLE write on ->power_on so the disable path is consistent
with the rest of the teardown. With this applied the same sequence is
harmless and the machine stays up; without it, it resets every time.

The unconditional write dates back to the original DP driver
(c943b4948b58 ("drm/msm/dp: add displayPort driver support")), but the
surrounding code has been restructured several times since, so no Fixes:
tag is offered.

Note that the eDP link-training failure that exposes this on the A16 is a
separate problem in the glymur eDP PHY and is reported separately; this
change is about not damaging the machine when training fails, for whatever
reason.

Tested on ASUS Zenbook A16 (UX3607OA), Snapdragon X2 Elite Extreme, on
linux-next next-20260803 and next-20260807. The machine has since been
running next-20260807 with this patch as its daily driver.

Patchwork: https://patchwork.freedesktop.org/patch/745167/