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

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]