drm/omapdrm: drop fb_debug_enter/leave

This is a no-op on atomic drivers because with atomic it's simply too
complicated to get all the locking and workers and nonblocking
synchronization correct, from essentially an NMI context. Well, too
complicated = impossible. Also, omapdrm never implemented the
mode_set_base_atomic hook, so I kinda wonder why this was ever added.

Drop the hooks.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-9-daniel.vetter@ffwll.ch