LCD won't unblank on "Dell Venue 10 Pro 5056" (Cherry Trail)

I recently got a "Dell Venue 10 Pro 5056" (from Ebay) w/ x5-Z8500 (Cherry Trail) and put Fedora on it.

The graphics works well (w/ X11 & wayland), but it has an odd symptom : after the screen saver blanks the LCD (not suspend), when I try to unblank it, the LCD is black. I can see the backlight come on, but the LCD is black. This happens every time. This happens w/ X11 & wayland. If I hook up HDMI, that unblanks OK.

Info :

uname -m : x86_64

uname -r : 5.9.8-200.fc33.x86_64

Linux distribution : Fedora !

Machine : Dell Inc. Venue 10 Pro 5056/0PK97J, BIOS 1.12.0 12/03/2019

intel_reg dump --all : intel_reg_dump.txt

I tried to dump the vbios, but got this error :

# cat /sys/devices/pci0000:00/0000:00:02.0/rom > vbios.dump
cat: '/sys/devices/pci0000:00/0000:00:02.0/rom': Input/output error

... and this message in dmesg :

[21368.134966] i915 0000:00:02.0: Invalid PCI ROM header signature: expecting 0xaa55, got 0xe98f

Thanks !