OvmfPkg/Gop: clear the screen to black in SetMode()

OvmfPkg/Gop: clear the screen to black in SetMode()

Today's implementation forgot to clear the screen to black in
SetMode(). It causes SCT SetMode() test fails.

The patch adds the clear screen operation in SetMode() to fix
the SCT failure.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>