drm/gma500: Remove dead code

This has the nice advantage that we'll get rid of a DRM_WAIT_ON user
for free.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>