drm/gma500: drop use of DRM_UDELAY wrapper

The DRM_UDELAY wrapper from drm_os_linux.h is used in a few places,
all other places calls udelay() with no wrapper.

There is no reason to continue to use this wrapper - so drop it
and direct call udelay().

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190519195526.3422-5-sam@ravnborg.org