Restore VIRGL_BIND_PREFER_EMULATED_BGRA macro

VIRGL_BIND_PREFER_EMULATED_BGRA macro was removed when the old BGRA emulation was removed, but that makes src/virgl_hw.h incompatible with Mesa, which still has a reference to the macro.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>