drm/gma500: use drm_modeset_lock_all

Only two places:
- suspend/resume
- Some really strange mode validation tool with too much funny-lucking
  hand-rolled conversion code.
- The recently-added lastclose fbdev restore code.

Better safe than sorry, so convert both places to keep the locking
semantics as much as possible.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>