drivers: cleanup meson build variable

drivers: cleanup meson build variable

The variable build is already initialized as true in
drivers/meson.build. Duplicate initializations can be removed from mlx.

When the variable build is set to false, it is easier to call
subdir_done() than branch the rest of the code on build condition.

Suggested-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Rosen Xu <rosen.xu@intel.com>