OvmfPkg: Don't build in QemuVideoDxe when we have CSM

OvmfPkg: Don't build in QemuVideoDxe when we have CSM

QemuVideoDxe installs its own legacy INT 10h handler for the benefit of
systems like Windows 2008r2 which attempt to use INT 10h even when booted
via EFI.

This interacts extremely badly with a CSM actually attempting to install
a real video BIOS.

The last thing done before invoking a legacy OpROM is to call INT 10h to
set a plain text mode. In the case where it's the video BIOS OpROM being
loaded, INT 10h will normally point to an iret stub in the CSM itself.

Unless QemuVideoDxe has changed INT10h to point to a location in the
0xC0000 segment that it didn't allocate properly, so the real OpROM has
been shadowed over them top of it, and the INT 10h vector now points to
some random place in the middle of the newly-shadowed OpROM.

Don't Do That Then. QemuVideoDxe doesn't do any acceleration and just
sets up a linear framebuffer, so we don't lose much by just
unconditionally using BiosVideoDxe instead when CSM is present.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190626113742.819933-4-dwmw2@infradead.org>