MdeModulePkg: HiiDatabaseDxe: HiiStringToImage() should not overwrite BltX

when has next line to draw, should not overwrite the BltX
to 0, instead should keep the BltX value that pass into
StringToImage function.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>