OvmfPkg/QemuVideoDxe: handle invalid BltOperation gracefully

According to the UEFI spec, EFI_GRAPHICS_OUTPUT_PROTOCOL.Blt() is supposed
to catch an invalid BltOperation, and report it with
EFI_INVALID_PARAMETER.

Remove the assertion from QemuVideoGraphicsOutputBlt() that prevents this
from working in NOOPT and DEBUG builds.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Rocky <xingrong.ni@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Reported-by: Rocky <xingrong.ni@intel.com>
Analyzed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=897
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

OvmfPkg/QemuVideoDxe/Gop.c

@@ -366,7 +366,7 @@ Returns:
 
   default:
     Status = EFI_INVALID_PARAMETER;
-    ASSERT (FALSE);
+    break;
   }
 
   gBS->RestoreTPL (OriginalTPL);