Minor regression with ramfb+gvt-g+edk2 with qemu 4.1

[Maxim Levitsky]

Hi!

I noticed that with qemu 4.1, when I try to open the
UEFI bios screen, the screen output is garbaged.

Basicaly what happens is this:

ramfb driver inside the uefi bios is hardcoded to initialize
the resolution to 800x600

Then when you enter the BIOS screen, it switches the resolution
to the 'preferred resolution' which by default is 640x480

However since commit a9e0cb67b7f4c485755659f9b764c38b5f970de4,
ramfb resolution is locked once it is set once, thus,
the switch to 640x480 is ignored.

Best regards,
        Maxim Levitsky