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)

drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h

@@ -35,6 +35,7 @@
 #include <drm/drm_dp_helper.h>
 #include <drm/drm_fixed.h>
 #include <drm/drm_crtc_helper.h>
+#include <drm/drm_fb_helper.h>
 #include <drm/drm_plane_helper.h>
 #include <linux/i2c.h>
 #include <linux/i2c-algo-bit.h>