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>