egl: Add MKDIR_GEN definition

egl: Add MKDIR_GEN definition

Adding linux-dmabuf Wayland protocol files as generated did the right
thing, by prepending $(MKDIR_GEN) so autotools didn't try to write into
a build directory which didn't yet exist.

Unfortunately MKDIR_GEN needs to be defined in every Makefile it's used
in (which we do now), or alternately defined and substituted in
configure.ac (which we don't do), and src/egl/ didn't actually have it
from either method. As unset variables expand to nothing, it was
silently being skipped.

Copy & paste the defintion to make sure drivers/dri2/ exists before we
try to generate files into it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reported-by: Nick Sarnie <commendsarnex@gmail.com>
Reported-by: Mike Lothian <mike@fireburn.co.uk>
Fixes: 02cc35937277 ("egl/wayland: Use linux-dmabuf interface for buffers")