drm/ast: Drop fb_debug_enter/leave

ast doesn't implement the mode_set_base_atomic hook this would need,
so this is dead code.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Sam Bobroff <sbobroff@linux.ibm.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: YueHaibing <yuehaibing@huawei.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190612091253.26413-2-daniel.vetter@ffwll.ch