Panel initialisation race condition on SDM845

Since ~v6.5 there is a random 60% chance that the display panel will fail to initialise during boot (with FBDEV_EMULATION), instead it will just be a black screen. The issue can be entirely mitigated by introducing some delay (adding console=ttyMSM0,115200 to the kernel cmdline is enough).

It happens on the OnePlus 6 and likely with other devices too. I have been able to reproduce it on upstream 6.5 with a small patch enabling prepare_prev_first in the panel driver.

Poking via modetest or blanking the display do nothing, and there is no additional messages printed via dmesg.

Cc: @lumag