drm/mga: drop dependency on drm_os_linux.h

Opencode all macros used from the deprecated drm_os_linux.h header file.
The DRM_WAIT_ON used 3 * HZ as timeout.
This was translated to 3000 msec.

The return value of mga_driver_fence_wait() was not
used, so make it return void to simplify code a bit.

v2:
- fixed timeout to 3000 msec (original value was 3 * Hz)
- drop unused return value from mga_driver_fence_wait()

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190623103542.30697-2-sam@ravnborg.org