drm: Move LEAVE/ENTER_ATOMIC_MODESET to fbdev helpers

This is only used for kgdb (and previously panic) handlers in
the fbdev emulation, so belongs there.

Note that this means we'll leave behind a forward declaration, but
once all the helper vtables are consolidated (in the next patch) that
will make more sense.

v2: fixup radone/amdgpu.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449218769-16577-3-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Thierry Reding <treding@nvidia.com> (v2)