OvmfPkg/QemuVideoDxe: Frame buffer config size may change in new mode

OvmfPkg/QemuVideoDxe: Frame buffer config size may change in new mode

https://bugzilla.tianocore.org/show_bug.cgi?id=339

The patch removes the assumption in QemuVideoDxe driver that it
wrongly assumes the frame buffer configure size is the same in
different video modes.
The assumption is true in old FrameBufferBltLib but is false in
new implementation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>