drm/debugfs: Fix framebuffer debugfs file init

The introduction of: drm/framebuffer: Add framebuffer debugfs file
broke vgem. That patch assumed that all drivers had initialized the
dev->mode_config.fb_lock mutex which happens in drm_mode_config_init().
vgem doesn't need to call drm_mode_config_init().

Fix this by only creating the framebuffer debugfs file for modesetting
drivers.

Fixes: 45d58b40292b ("drm/framebuffer: Add framebuffer debugfs file")
Patch-mainline: 20171113231201.26177-1-noralf@tronnes.org @ 13/11/17, 11:12 p.m.
Link: https://patchwork.freedesktop.org/patch/msgid/20171113231201.26177-1-noralf@tronnes.org


Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
[stalek@codeaurora.org:
1. add missing change due to code conflict
2. drop function call change due to conflict.].

Change-Id: I8a6afb8a47c0f85b03b6148377fb6d120a77c099
Signed-off-by: Shubham Talekar <stalek@codeaurora.org>