gallium: fix type of flags in pipe_context::flush()

It should be unsigned, not enum pipe_flush_flags.

Fixed a build error:

  src/gallium/state_trackers/egl/android/native_android.cpp:426:29: error:
  invalid conversion from 'int' to 'pipe_flush_flags' [-fpermissive]

v2: replace all occurrences of enum pipe_flush_flags by unsigned

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>

[olv: document the parameter now that the type is unsigned]

src/gallium/drivers/rbug/rbug_context.c

@@ -942,7 +942,7 @@ rbug_clear_depth_stencil(struct pipe_context *_pipe,
 static void
 rbug_flush(struct pipe_context *_pipe,
            struct pipe_fence_handle **fence,
-           enum pipe_flush_flags flags)
+           unsigned flags)
 {
    struct rbug_context *rb_pipe = rbug_context(_pipe);
    struct pipe_context *pipe = rb_pipe->pipe;